add coverage tools to habitrpg-shared

This should help with #174, though it only has coverage for the .coffee files, not the pure js.  As far as I can tell, though, that's all we're actually testing now.

I spent a while trying out some other tools for coverage, but ibrik couldn't process some of the files and karma-coverage seems to use ibrik to preprocess coffeescript.  Nothing seemed to cover both the js and the coffeescript well.

This is very easy to use - `npm run coverage` will generate a coverage.html file with nicely formatted coverage information.

I'm new to node, so please let me know if I'm doing something weird or violating best practices in some way.