Augment iAudioTrack with useful state change events

[codinronan]

Basically, for change detection reasons my app needs access to these state changes so that I don't have to use setInterval. I need to be able to implement the OnPush change detection strategy, which is extremely hard when a setInterval is needed to detect changes to these objects.

The following events are added to IAudioTrack and implemented in both classes:

  onLoaded: Subject<IAudioTrack>;
  onPlaying: Subject<IAudioTrack>;
  onStop: Subject<IAudioTrack>;
  onPause: Subject<IAudioTrack>;
  onFinished: Subject<IAudioTrack>;
  onProgressChange: Subject<IAudioTrack>;
  onError: Subject<IAudioTrackError>;

...

export interface IAudioTrackError {
  track: IAudioTrack,
  error: ErrorEvent,
}

This also incorporates #115 so you'll see the same set of changes.

@arielfaur let me know what you think. I'm not convinced this is the best implementation but it is the most obvious.

[proustibat]

@codinronan thanks for reviewing my intents. This is exactly what I wanted.
@arielfaur let us know if theses changes are ok for you