Refactored music player and song queue handling

* DetailViewController.cs: DetailViewController now handles the song
  queue. MusicPlayer is now just a single song player that handles all
  playback details of the current song, but handling prev and next
  song requests now takes place here. 

* MasterViewController.cs: Moved methods to get artist by index, song
  by index path, etc, to static Songs class.

* MyMusicPlayer.cs: Now just handles playing one song at a time at the
  DetailViewController's request. So AVQueuePLayer is changed to
  AVPlayer. Getting the next and previous songs is relegated to the
  DetailViewController, handling remote events for NextTrack and
  PrevTrack are now handled there, however this still handles the
  scrubbing. 

* Songs.cs: Added helper methods to overcome a Dictiionary's non
  indexable nature. Makes the MasterViewController code a bit cleaner