Added search functionality.
Searches on artist and song

* ArtistSongTableViewSource.cs: Separated the Source from the
  UITableViewController so the search controller could use it too. 

* LockScreenAudio.csproj: 

* MasterViewController.cs: Added search bar and search weak delegate.

* Songs.cs: Songs is now responsible for testing if the user is
  searching and returning the appropriate results. .