Feature detection for w3c

This is a first crack at making Howler aware of Firefox's more spec-compliant implementation of the w3c We Audio Api spec. There are a number of changes to the spec discussed in the deprecation section that webkit hasn't implemented:

https://dvcs.w3.org/hg/audio/raw-file/tip/webaudio/specification.html#DeprecationNotes

Because Mozilla is implementing the spec right now, the current spec is being followed.

Note: you will need to download Firefox Nightly ( https://nightly.mozilla.org ) in order for any of this to work at all currently, and there are a number of bugs that need to be resolved for it to work well. The tracking bug is here:

https://bugzilla.mozilla.org/show_bug.cgi?id=779297