add sprockets-browserify integration, remove js in /vendor with npm packages

@DrTom please review this strategy before I go further in this direction. *Not urgent.*

- we can `require(js)` with proper encapsulation etc instead of just concatenating `//= require js` stuff
- everything is integrated with sprockets, rails works as before (no scripts/hacks to run)
- `/vendor/javascripts` is now `/node_modules` (yes it is a new top-level dir but will save us much headache when using the scripts outside the app (uni tests(!), helper scripts, …)
- configured to support coffeescript

Questions? Notes?

![](https://pbs.twimg.com/media/BwH0jbOIMAEvXhX.jpg)