Pull api updates

* Changed linkRelations() to listPullRequests() and mergability() to updatePullRequest() and added missing parameters.
* Left  linkRelations() and mergability() for backwards compatibility (just in case).
* Inheriting GitHubFullPull from GitHubPull.
* Added missing data members to GitHubPull and GitHubFullPull.
* GitHubPullBranch is a new object that represents the 'head' and 'base' branch points of a pull request.

@AviBueno, getAttributes should be protected.