Added support for custom notification sounds

This change assumes that notification sounds should be handled on the level of the Cordova plugin to line up functionality with iOS.
The file name of the audio file is taken from the "sound" property of the push payload. This should be, for example, "/res/alert.mp3" if the file is located in "www/res/alert.mp3".
org.apache.cordova.media is required to play the sound.