Jpsullivan/pull requests api

CRUD functionality for Pull Requests is in place.  As of now, the only things missing from this are:

- Fetching commits within a pull request
- Merging pull requests
- Determining "mergibility" of a particular pull request
- Integration tests for the PullRequestsClient class

Otherwise, to me this seems like a good starting point.  Don't hold back if I screwed up something! :thumbsup: