Add Coffeescript

Hi,

I know there's already a pull request to add Coffeescript, but this one does it properly without the other commits. It also differs in that it precompiles the Coffeescript.

It …

* … adds the `coffee-script` gem and requires it in `app.rb`
* … adds rake tasks similar to the styles ones
* … converts application.js to coffeescript, so it's not lost when you run the rake tasks