Return the activity with the given id.

        Note that this contains more detailed information than returned
        by `get_activities`.

def get_activity(self, id_num):
        """Return the activity with the given id.

        Note that this contains more detailed information than returned
        by `get_activities`.

        """
        url = self._build_url('my', 'activities', id_num)
        return self._json(url)