Support Firefox with unprefixed AudioContext

I was excited to run your examples in Firefox, which in version 23 (now alpha) has partial support for Web Audio. However, it'll only work if you try using unprefixed `AudioContext`, which this patch does.

A possible future change for fuller compatibility would be to replace the MP3 files with Oggs, which both Firefox and Chrome support. When I do so with the [ScriptProcessor node example](http://webaudioapi.com/samples/script-processor/), I get identical results in both browsers.