Build mopidy.js with browserify

Happy new year!

I'm finally happy with this browserified version of mopidy.js that I'm submitting a pull request. If I've missed anything out, please let me know!

I just want to say that mopidy is an amazing project and everyone in the office is so thrilled to be using it.

To summarise: This PR contains the 4 commits to allow browserify to build the browser version of mopidy.js as part of issue #609 

Thanks,
connrs.