Allow Switch back to MODE_NORMAL

Howdy,

I've been testing this plugin and it works great. However, there is one issue on Android.

The problem is that neither `MODE_IN_COMMUNICATION` or `STREAM_MUSIC` is the normal media mode. So after switching to one or the other, audio from other applications does not function correctly. We need to be able to switch back to the communication mode when the application pauses or ends: `MODE_NORMAL`

http://developer.android.com/reference/android/media/AudioManager.html#MODE_NORMAL

Will work on adding some methods to do that.