'Mark all notifications in this repository as read.
:param str last_read: (optional), Describes the last point that
notifications were checked. Anything updated since this time will
not be updated. Default: Now. Expected in ISO 8601 format:
``YYYY-MM-DDTHH:MM:SSZ``. Example: "2012-10-09T23:39:01Z".
:returns: bool'

@requires_auth
def mark_notifications(self, last_read=u''):

    url = self._build_url(u'notifications', base_url=self._api)
    mark = {u'read': True}
    if last_read:
        mark[u'last_read_at'] = last_read
    return self._boolean(self._put(url, data=dumps(mark)), 205, 404)
