Adds MutationObserver-shim as a dependency, fixes #646.

As promised [here](https://github.com/masayuki0812/c3/issues/646#issuecomment-65675358), MutationObserver-shim added to dev dependencies. I've simply concatenated it to the front of the distributable inside of a jshint ignore block; don't know whether there's a better way of doing that.