title: How to detect changes to PHAuthorizationStatus?
body: <p>I need to detect changes to my app's permission to access Photos using PhotoKit on iOS 8. I can get the status with <code>PHPhotoLibrary.authorizationStatus()</code>, but I need to know when this status changes, so I can update the UI (which is asking them to change the permissions).</p>
  
  <p>If the user launches the app, I check the <code>authorizationStatus</code> in <code>viewDidLoad</code>, then display the message if the status is <code>.Denied</code> or <code>.Restricted</code>. But if the user then goes to Settings and grants access, when they return to the app it is still showing that message. I need to handle those status changes - if it changes to <code>.Authorized</code> I need to remove the message and reload my collection view.</p>
  
  <p>I couldn't find any notification I could subscribe to. How does one accomplish this?</p>