This adds logic so that Short* classes can return the fuller
representation. For example:

    >>> import github3
    >>> users = [(u, u.refresh()) for u in github3.all_users(10)]
    >>> for short_user, user in users:
    ...     assert isinstance(short_user, github3.users.ShortUser)
    ...     assert isinstance(user, github3.users.User)

Will run without exceptions.

Related to gh-670