title: Tracking is user online via parse.com
body: <p>I'm developing an app using back end from parse.com and I got a question and in docs or in google couldn't find the answer how to track if user is online or not?
  My best idea is to create callback for listening while app is opened and remove it in <code>onStop()</code> (I think in <code>onDestroy()</code> is not a good way, right?) and then send it to one of columns of <code>ParseUser</code> user accordingly <code>true</code> or <code>false</code>.
  But I'm not sure that parse.com doesn't support it, maybe I'm blind and there is a documented way to do that?
  Thanks in advance.</p>