Pull request for #514

@todo #115:1hr Implement the Git Data References API. We need to
     *  implement the following:
     *  1) Add the operations in http://developer.github.com/v3/git/references/
     *  to the References interface.
     *  2) A class RtReferences, which performs RESTful request operations
     *  against the References API.
     *  3) A class MkReferences, which mocks the References interface.
     *  4) An interface Reference, which represents a single Reference of the
     *  Git Data API.
     *  5) RtReferences, a Reference implementation which fetches and modifies
     *  Reference information according to the References API, and
     *  6) MkReference, which mocks a single Reference object.
     *  Don't forget to implement unit tests and integration tests.