Pull Requests API Implementation

Attempting to take a stab at #135.

Basic functionality for Pull Requests is in place.  As of now, the only things missing from this are:

- [x] Fetching commits within a pull request
- [x] Merging pull requests
- [x] Determining "mergibility" of a particular pull request
- [x] 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: