Added convenient script for generating changelog draft.

[haberman]

Review to @TeBoring.

[haberman]

Sample output:

$ ./generate_changelog.py v3.0.2
C++
 - 58860c0 Merge remote-tracking branch 'origin/3.0.x' into merge
 - 08b1c71 Fix #2032 unused parameter 'deterministic'
 - 7c3f7c6 Fix #1955 clang-tidy warning misc-macro-parentheses
 - df6088a detect invaild JSON encoding in bytes field
 - 5a17660 detect invaild JSON encoding in bytes field
 - e514f23 fix #1342 cause by ownership issues
 - eedc7be Restore New*Callback into google::protobuf namespace since these are used by the service stubs code
 - c59473d Merge pull request #1044 from mark-whiting/master
 - 169d0ca Merge pull request #1893 from google/3.0.0-GA
 - c466f4b Be consistent with the use of CHECK()/ GOOGLE_CHECK().
 - fd1c289 Adding missing generic gcc 64-bit atomicops.

Java
 - 9ac84f8 Fix erroneous comment regarding String.substring
 - 58860c0 Merge remote-tracking branch 'origin/3.0.x' into merge
 - 1fc416b Allow the JsonFormat.Parser to ignore unknown fields
 - 08b1c71 Fix #2032 unused parameter 'deterministic'
 - 7c3f7c6 Fix #1955 clang-tidy warning misc-macro-parentheses
 - b964976 Merge pull request #2024 from pstavirs/master
 - b6dec9b update the OSGi SymbolicName and ExportedPackage for the javanano library
 - eedc7be Restore New*Callback into google::protobuf namespace since these are used by the service stubs code
 - 30647ca Use 64-bit protoc binaries in compatibility tests.
 - 169d0ca Merge pull request #1893 from google/3.0.0-GA

Python
 - b6dec9b update the OSGi SymbolicName and ExportedPackage for the javanano library

JavaScript
 - 58860c0 Merge remote-tracking branch 'origin/3.0.x' into merge
 - 7645a3d Merge pull request #1884 from hochhaus/valueWriterFn
 - ea081fe Fix missing import of jspb.Map (#1885)
 - 6b3d120 Merge pull request #1887 from sheffatguidance/fix-js-api-documentation
 - 1112989 Merge pull request #1884 from hochhaus/valueWriterFn
 - 169d0ca Merge pull request #1893 from google/3.0.0-GA
 - 9a11ab4 Fix Issue #1869: faulty js API documentation
 - a217408 Fix valueWriterFn variable name

PHP
 - 3b001ca Some php engine implementation doesn't have return_value_ptr properly set. Explicitly use &return_value.
 - c6fa9c7 Auto-generate proto files for tests.
 - f174d36 Add back missing test proto files.
 - e0e5466 Check in php implementation. (#2052)

Ruby
 - 05aa0df Fix hash computation for JRuby's RubyMessage
 - b5bbdb0 Merge pull request #2037 from abscondment/fix-2036-ruby-hash
 - 58860c0 Merge remote-tracking branch 'origin/3.0.x' into merge
 - de02863 fix #2036: use `rb_hash_*` to accumulate hashes
 - 047419a failing test for #2036:
 - 3a674ff upb bugfix: JSON map entry keys were passing the wrong closure.
 - d4213d8 Ruby: make sure map parsing frames are GC-rooted.
 - 866d3e5 Fixing regular expression...
 - 8d8115b Merge pull request #1878 from haberman/rubywkt
 - 3cec2ea Ruby: added custom Struct exception type and fixed Makefile.am.
 - 169d0ca Merge pull request #1893 from google/3.0.0-GA
 - a207a2b Fix for JRuby (assert_true is not present).
 - e3094a8 Ruby: added API support for well-known types.

Csharp
 - 58860c0 Merge remote-tracking branch 'origin/3.0.x' into merge
 - 9b8da10 Rm check on dependency in the C# reflection API (#2051)
 - e389165 Add more JSON tests around underscores (#1963)
 - f9d93f3 Regenerate conformance files to include extra oneof fields.
 - 169d0ca Merge pull request #1893 from google/3.0.0-GA
 - be78976 Merge pull request #1861 from jskeet/fix_to_camel_case
 - a8aae89 Bring C#'s ToPascalCase method in line with C++. (This still doesn't fix the conformance tests, but at least we're now consistent with the C++ code.)
 - e672047 Remove legacy_enum_values flag for GA.

Objective C
 - 6ab51a0 Use a custom dictionary to avoid NSNumber operations.
 - 5904279 Remove the custom key functions and just use the system provided defaults.
 - 9c4be5f Merge pull request #2112 from pherl/merge
 - 1aa6500 Update the ObjC version checks to support a min and current version.
 - 58860c0 Merge remote-tracking branch 'origin/3.0.x' into merge
 - 53387e5 Merge pull request #2090 from guoxiao/find
 - 7377eb2 Merge pull request #1970 from thomasvl/objc_any_helpers
 - 5d35e60 Merge pull request #2094 from thomasvl/update_wkt_comments
 - 5699b92 More complete nil/reset tests within a oneof
 - 708296e Fix some constants to be correct for the message class in use.
 - 161b937 Fix error and add note about lossy issues
 - 14e74f6 Support the -Wassign-enum compiler flag. (#2085)
 - 337ec30 Add ObjC helpers for Any WKT.
 - 82133ba include std::find()
 - 13a4124 Make Root's +extensionRegistry generation smarter.
 - 78a6d31 Speed up ObjC Generation with large dependency trees
 - 2e66a61 Support GenerateAll().
 - 290d26b Remove the compiler options from ImportWriter.
 - 93362a5 Move the ImportWriter into the ObjC Helpers.
 - ff2a660 Adds better support for protos without packages  (#1979)
 - e505098 Adding casts so that code importing protobug using -Wconversion does not generate warnings.
 - a248420 Fixes extra whitespace on generated comments. (#1950)
 - ff85a17 More complete nil/reset tests within a oneof
 - a0df678 Fix some constants to be correct for the message class in use.
 - 17d601a More explicit tests for nil behaviors on fields.
 - ca5b775 Record zero for "has" for proto3 if in a oneof.
 - 1a6c1d0 Never use strlen on utf8 runs so null characters work.
 - 237f321 Adds support for appledoc in generated code.  (#1928)
 - 32fadc0 Migrating documentation of the ObjectiveC runtime code to appledoc. (#1867)

To view a commit on GitHub: https://github.com/google/protobuf/commit/<commit id>

[haberman]

cc @xfxyjwf @pherl

I don't think the test failures are related.

[xfxyjwf]

LGTM

I canceled most travis runs as they are very slow and blocking 3.1.x release tests.