Add Audio Support for Emscripten

This adds an imlementation of the SimpleAudioEngine class in Cocos2DX via Emscripten's SDL implementation. This in turn is implemented with HTML5 audio elements.

In the future, we should look at this again, and probably replace with a WebAudio based implementation. But for now, this is the approach that will get us onto the most browsers soonest. Only operation I couldn't figure out how to implement was rewind, as this seems not to be exposed in SDL.