'Fork this gist.
:returns: :class:`Gist <Gist>` if successful, ``None`` otherwise'

@requires_auth
def fork(self):

    url = self._build_url(u'forks', base_url=self._api)
    json = self._json(self._post(url), 201)
    return self._instance_or_null(Gist, json)
