Provide faster alternative for setImmediate with MutationObserver.

I noticed some faster ways are available to do this. Since the browser vendors aren't to excited to add a `setImmediate` method, it needs a good alternative.

Performance this test: http://jsperf.com/setimmediate-test/14
Browser support: http://caniuse.com/#feat=mutationobserver

Not my code, just copied it from the jsperf and did some small readability improvements.

Discussion of the Chrome team: https://code.google.com/p/chromium/issues/detail?id=146172
Discussion of the Mozilla team: https://bugzilla.mozilla.org/show_bug.cgi?id=686201
The source check can also be removed?