Add a bit of functionality to Crafty.audio

This makes it so that `Crafty.audio.play` returns the actual audio element used to play the sound.

I also fixed a bug in how `audio.stop()` works; it wasn't properly updated when we switched to the channels approach.

There are probably some other, simple things we could do to make the audio stuff easier to use; since we might switch to using the web audio API in the future, it would be ideal if you never *needed* to manipulate the audio elements directly.

So anyone have any specific ideas for small improvements?