title: SWRevealViewController is nil when adding gesture recognizer
body: <p>I'm working on an app that will make use of the SWRevealViewController class. The app works so far, in that I'm able to tap the left bar button item to bring out the rear view controller, but when I add a gesture recognizer in my front view controller, the SWRevealViewController is nil. I have no clue why this is so any help would be appreciated. </p>
  
  <pre><code>//not nil here. 
  SWRevealViewController *revealController = [self revealViewController];
  //somehow it becomes nil on the very next line and from then on I can't hold the reference to it
  [self.navigationController.navigationBar addGestureRecognizer:[revealController panGestureRecognizer]];
  
  UIBarButtonItem *revealButtonItem = [[UIBarButtonItem alloc] initWithImage:[UIImage imageNamed:@"reveal-icon.png"]
                                                                       style:UIBarButtonItemStyleBordered target:revealController action:@selector(revealToggle:)];
  </code></pre>