Flash fallback breaks on IE8/FF using CommonJS

Hey @zohararad, and thanks for this super-useful module!

The flash fallback is currently broken when loading `audio5.js` CommonJS-style (and possibly AMD-style too), due to a bug with the swf player not being able to get the references the audio player instances.

This patch fixes this issue by replacing `Audio5js.flash` with a more explicit global (`window.__audio4js_flash__`), so that the swf can get a hold on the instance passed to it.

I'd be very happy if you merge this in and do a version bump + npm publish

Oh, and btw this project badly needs tests (but you are probably aware of that ;-)

Cheers