Pull api updates - Githubissues

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.

tan-tan-kanarek / github-php-client