Added set/getVolume, fadeIn, fadeOut; Only for iOS

Four new methods were added:

getVolume():Number  - returns player volume from 0-1
setVolume(vol:Number) - sets player volume, 0-1
fadeOutSong(fadeTime:Number):void
fadeInSong(fadeTime:Number):void

The fade functions take a fadeTime in seconds.

Fix for iOS - use AVAudioPlayer when it can, otherwise
MPMusicPlayerController (which does not allow volume changes).
Final changes to new volume functions. See readme.