Stop tracking current activity. end_time can be passed in if the
        activity should have other end time than the current moment

def stop_tracking(self, end_time = None):
        """Stop tracking current activity. end_time can be passed in if the
        activity should have other end time than the current moment"""
        end_time = timegm((end_time or dt.datetime.now()).timetuple())
        return self.conn.StopTracking(end_time)