Make top-level script JS

Switches out CoffeeScript with JS for the Gruntfile.coffee and app.coffee scripts for the reasons explained [here](http://stackoverflow.com/a/19617103/1601953).

However, Grunt includes coffee-script as a dependency, so maybe the commit related to that could be omitted. I'll test if `grunt` works with Gruntfile.coffee without `coffee` installed globally, when I have a bit more time.