Load skins from original WSZ files instead of individual BMPs

Loads WSZ files using js-zip ( http://stuk.github.io/jszip/ ) which means it now has a dependency, but loading original skin files is pretty cool :)

Currently it's rendering a little funny. Not sure what that is.

![image](https://cloud.githubusercontent.com/assets/622455/5060631/5b09f40c-6db5-11e4-8e40-b3f1887d7165.png)

Feel free to rewrite or ignore this!

Could we add this back in, but have it fetch the file via a relative URL like `skins/base-2.91.ws`. Would that work?