Try unprefixed AudioContext (for Firefox support)

It makes me sad when Web Audio demos don't work in Firefox, so I patched this to work. The only change is to try `AudioContext` (standard name) before `webkitAudioContext`, which is the name Chrome's developers assigned it when it was still an experimental feature.

[Julia's blog post](http://jvns.ca/blog/2013/11/25/day-33-how-to-make-music-with-clojurescript/) brought me here, btw :)

If you copied/learned the `webkitAudioContext` bit from some demo or tutorial, can you link me to it so I can send them a pull request too?