title: is there any way to get last activity from CMMotionActivityManager
body: <pre><code>-(void)queryActivityStartingFromDate:(NSDate *)start
                             toDate:(NSDate *)end
                            toQueue:(NSOperationQueue *)queue
                        withHandler:(CMMotionActivityQueryHandler)handler;
  </code></pre>
  
  <p>This API may return empty activities if startDate and endDate very close. </p>
  
  <p>I wanna get activity in timestamp. </p>