Add sinatra asset pipeline

This branch:

- Adds the [sinatra-assets](https://github.com/rstacruz/sinatra-assetpack) and uglify gems, to support minification of js and css.
- Moves the front-end to `app/js` and `app/css`, the convention for sinatra assets.
- Removes an extra main.css file that snuck in somehow during the merge.

I don't know ruby, so be gentle when explaining all the ruby-isms that I've violated.