title: Want to change NavigationItem title
body: <p>I've embedded my first view into a NavigationController and then i set the segue as push.
  I want to change in each view the title of the Navigation bar to display something and update it through my code.
  i tried to call the navigation controller like this:</p>
  
  <pre><code>self.navigationController.navigationBar.topItem.title = @"YourTitle";
  </code></pre>
  
  <p>but this wont work, how can I do it?</p>