Polyfill MutationObserver

Currently, MutationObserver is being used, but not polyfilled.

This will fail in versions of IE < 11 (what versions IE and other browsers does scribe officially support).

It will also fail in PhantomJS, which is using a slightly outdated version of Webkit, (MutationObserver support [is planned for PhantomJS 2.0](https://github.com/ariya/phantomjs/issues/10715#issuecomment-50734088), which has not been released yet) so anyone who is running tests with scribe via PhantomJS will currently see errors.

The polyfill used here is cribbed from [megawac/MutationObserver.js](https://github.com/megawac/MutationObserver.js).