window.postMessage for a faster runloop?

So I was thinking about how heavily Ember uses the runloop and that the timer resolution varies so incredibly wildly between browsers, sessions, star-alignment, etc and many modern browsers still have a 10ms hard min (or "clamp").

I had previous seen pretty neat things done with using `postMessage` as a way to have an almost immediate `defer`, so thought, why can't backburner use this, when it's available?

This is more of an proof of concept example then an actual PR. Doesn't include fallback to `setTimeout` and  didn't put the code in any particularly thought out place.

I just wanted to start a dialog and see if you peeps had tried this or discussed?

Btw--I am aware of issues with a couple browsers, notably [IE8](http://stevesouders.com/misc/test-postmessage.php).