Added basic playlist support

I've added audio support for a playlist, meaning playing a list of sounds one after another. This could be useful to create a playlist of a game soundtrack. 
This looks wrong. The callback shouldn't be invoked when passed to the event listener. Also listener should be removed after use. Should look like this:

            var endedListener = function(){
              Q.audio.channels[i].channel.removeEventListener('ended', endedListener);  
              callback();
            }
            Q.audio.channels[i].channel.addEventListener('ended', endedListener);