Added mustache template and cleaned up js/script.js

Mustache templating seemed to make sense here and clean up the really long line in `js/script.js`. I also changed it so it would pass my default `jshint` tests, I can give you that file if you would like.

Also added `grunt-contrib-concat` so `jquery` + `mustache` + `js/script.js` can all go into the same file (`bundle.js`) and changed the link in `index.haml`. You should probably consider managing/installing your front end dependencies in a way that means they can be updated easily. Maybe something like [browserify](https://github.com/substack/node-browserify). Probably a bit overkill to add that as a dependancy for this though, its kind of big.