title: how to implement pusher presence authentication callback in javascript
body: <p>I have set my pusher object for the destination authentication request</p>
  
  <pre><code>Pusher.channel_auth_endpoint = '/php/pusher_auth.php';
  </code></pre>
  
  <p>Is here a js callback event so that I could display an ajax loading gif while pusher is authenticating and show a div after pusher auth is successful and done.</p>