Switching from nose to py.test.

Also removing all references to nose, and in the
process just making the CONTRIBUTING doc point to
the main project.

Also adding `grpcio` as a hard dependency.

Author: dhermes

.coveragerc

@@ -1,4 +1,8 @@
+[run]
+branch = True
+
 [report]
+fail_under = 100
 show_missing = True
 exclude_lines =
     # Re-enable the standard pragma

.gitignore

@@ -26,7 +26,6 @@ pip-log.txt
 # Unit test / coverage reports
 .coverage
 .tox
-nosetests.xml
 
 # Translations
 *.mo

.travis.yml

@@ -8,6 +8,7 @@ script:
   - tox -e py27
   - tox -e py34
   - tox -e lint
+  - tox -e cover
   - tox -e system-tests
 
 after_success: