move `self.delegate?.playerPlaybackStateDidChange(self)` and `self.delegate?.playerBufferingStateDidChange(self)` to value observer `didSet`.
move `self.delegate?.playerPlaybackStateDidChange(self)` and `self.delegate?.playerBufferingStateDidChange(self)` to value observer `didSet`.

piemonte/Player,piemonte/Player,piemonte/Player