Add cjsx to the Asset Pipeline

Uses [coffee-react](https://github.com/jsdf/coffee-react) to process `app/assets/javascripts/example.js.cjsx`, which was mentioned in #44.

This is a hack since it depends on the `cjsx` executable, but it _works_, which is a start. If this were cleaner (maybe vendoring in coffee-react), would there be interest in merging it?

I think there's a good case for it -- CoffeeScript is baked into Rails and `cjsx` is a great way to write React in CoffeeScript.