ios support play background and native media control bar

1. support ios native media control panel on lock screen and left of multi-task bar
2. continue play audio after lock screen with dark screen without stop.
3. support play audio in background (need user tap the play button in the media control panel of ios after xbmc enter background state)
4. support continue play after ios audio interrupt, like an alarm message form other app, or a phone call
5. support show current playing item info on ios' lock screen, thumb also supported.

do not support play video background yet (when user init the video play/continue, default will pause video when enter background), since I not known how to drop frames.