Use Browserify

Current situation:

* development: `coffee --watch` over many small files + PHP logic to load the files separately
* production: custom build script + PHP logic

With [Browserify](http://browserify.org/):

* development: `browserify --debug main.js > bundle.dev.js`
* production: `browserify main.js > bundle.js`

Unfortunately, Browserify doesn't have a `--watch` flag.

Inspiration: http://esa-matti.suuronen.org/blog/2013/03/22/journey-from-requirejs-to-browserify/