title: Issue with requesting the video thumbnail using MPMoviePlayer
body: <p>I am trying to parse the JSON data which has video urls and the title. After parsing the data I am showing the video thumbnail in a UITableviewCell. The problem is that when the data is parsed and when its time to request for the thumbnail the NSNotification selector method is never been called. Here is my code:</p>
  
  <pre><code>override func viewDidLoad() { 
  super.viewDidLoad() 
  getJson_results() 
  }
  
  override func viewWillDisappear(animated: Bool) {
  NSNotificationCenter.defaultCenter().removeObserver(self)
  }
  
  func videoThumbnailIsAvailable(notification: NSNotification) {
      println("Flag1") //the control never reaches here
      var thumbnail = notification.userInfo?[MPMoviePlayerThumbnailImageKey] as UIImage
      self.temp_image = thumbnail
  }
  
  
  func videodurationIsAvailable(notification: NSNotification) {
      var val = moviePlayer?.duration
      self.temp_duration = val!
  }
  
  override func tableView(tableView: UITableView, numberOfRowsInSection section:    Int) -&gt; Int { 
      return length_array; 
  }
  
  override func numberOfSectionsInTableView(tableView: UITableView) -&gt; Int {
  return 1
  }
  
  override func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -&gt; UITableViewCell {
      let cell = tableView.dequeueReusableCellWithIdentifier("Cell", forIndexPath: indexPath) as WPR_Main
      cell.imgView?.image =           get_Media_thumbnail(NSURL(string:self.url[indexPath.row])!)
      cell.titleLabel?.text = self.titles[indexPath.row]
      return cell
      }
  
  
  func get_Media_thumbnail(url: NSURL) -&gt; UIImage {
      moviePlayer? = MPMoviePlayerController(contentURL: url)
      NSNotificationCenter.defaultCenter().addObserver(self,
      selector: "videoThumbnailIsAvailable:",
      name: MPMoviePlayerThumbnailImageRequestDidFinishNotification,
      object: nil)
      moviePlayer?.requestThumbnailImagesAtTimes([1.0], timeOption:.NearestKeyFrame )
     return self.temp_image
  }
  </code></pre>