Add support for URI and UUID attribute types

[protocool]

Summary of Changes

This adds support for generating accessors for NSURIAttributeType and NSUUIDAttributeType attributes, which were introduced in iOS 11 and macOS 10.13.

With the introduction of this change, the mogenerator binary must be built using Xcode 9.

[protocool]

WRT the CI failure, that may be the result of my changing test/Rakefile to query for the latest SDK instead of using a hard-coded path to the 10.12 SDK.

I can amend to point to the relevant path for the CI server, just let me know.

[rentzsch]

Hey Trevor,

Thanks for the Pull Request!

I do prefer the explicitness of specifying the entire hard-coded path in the Rakefile's OSX_SDK.

Also, you probably have to update the osx_image in .travis.yml to xcode9 to get CI to pass (assuming Travis already has and supports Xcode 9).

[protocool]

Hey. I've restored the specific path (to 10.13 SDK) and, as you suspected, it was trivial to update the travis config to use Xcode 9.

Let me know if there are any other concerns.

[tomekc]

So... will this PR be pulled or stay forever here...? :)

Edit: never mind, I integrated in with my fork. ;-)