Move play list control into a seperated class.
1. DZAudioPlayer now acts as a player only, not related with feed item.
2. play list control is now in DZPlayList, maintaining the current play list.
3. add event PlayerWillAbortPlaying when a playback will abort.
4. play the next item in the play list when one is complete.
[TODO]
when the play view controller is shown, auto play the next will not change the display correctly,
especially in the feed view controller.