'Get an annotated tag.
http://learn.github.com/p/tagging.html
:param str sha: (required), sha of the object for this tag
:returns: :class:`Tag <github3.git.Tag>`'

def tag(self, sha):

    json = None
    if sha:
        url = self._build_url(u'git', u'tags', sha, base_url=self._api)
        json = self._json(self._get(url), 200)
    return self._instance_or_null(Tag, json)
