react-rails gem

Fixes #231

There's still a bunch of work to do, but the important things pretty much just work!

This depends on a couple things, namely that we release a react-source v0.4.1 with #217. I built that locally and installed it so I could test.

This is another thing that I would like to pull out of the react repo and publish independently, but for the time being this lets us move fast. I may actually just do that before this is ready to merge in anyway...