Minify all the CSS to one file

This uses [Grunt](http://gruntjs.com) and [CSSMin](http://github.com/gruntjs/grunt-contrib-cssmin) to minify all the CSS into one file. This eases the build process in the future, as we can now use any [Grunt plugin](http://gruntjs.com/plugins), and it's also one less HTTP request.

Helps out #5.