Don't set albumArt to nil after delivering notification. #10

Instead, set it to nil on play state change if the track has changed. This should make album art available to notifications delivered on shortcut presses.
Merge branch 'master' of github.com:citruspi/Spotify-Notifications

* 'master' of github.com:citruspi/Spotify-Notifications:
  Don't set albumArt to nil after delivering notification.
Re-organized code for getting album art into SNXTrack class
Fixed album art fetch conditional