Fix Safari audio issues

Use HTML5 feature detection instead of user-agent sniffing to determing audio strategy.
(see http://diveintohtml5.info/everything.html for more info on the technique used)

Works on Safari 5+ (and should work on any other browsers which support HTML5 audio)

I didn't check for codec support because anything that supports HTML5 audio is pretty much guaranteed to support one of the three provided formats.

Fixes #1