Save data when app will terminate.
1. close player and save database in DZAppDelegate.
2. add save method in DZAudioPlayer.
Add filter function to feed item table view.
1. connect outlet for the segmented control.
  control and action outlets linked in DZFeedHeaderCell class.
  add view controller property in DZFeedHeaderCell linked to DZFeedViewController.
  trigger view controller's filtering method in header cell.
2. add segue for now playing button in the navigation bar.
3. remove unused reference to play-button-highlight.png in the storyboard file.