Wrap AudioContext creation in try/catch

When creating an AudioContext, an error may be thrown due to there
being a limit on the number of hardware contexts that can be created.

This wraps the AudioContext contstructors in a try/catch, and tries
to fall back to HTML Audio as if Web Audio was not available.

I noticed this issue when opening a number of windows containing Howler in Chrome 33. Strangely, I didn't see the issue when opening tabs instead of windows.

Example JSFiddle: http://jsfiddle.net/cbuckley/vG4xy/
Chromium issue: [308784](https://code.google.com/p/chromium/issues/detail?id=308784)