Fix #681: Modularize css

This splits style.css up into multiple modular files, sticks them in `public/css/src`, and tells grunt to concatenate them into `public/css/prod/jsbin-<version>.css` every build.

This is a bit crude, as the css files are concatenated alphabetically (their order in `public/css/src`), but it works.

This branch should probably be modified to order those css files in a similar way to how the JS files are ordered using scripts.json.

I thought I'd see what you think before creating a style.json which would serve a similar purpose for the CSS.