`react-core` npm module

Jordan wanted an easy way to use React, and specifically behave nicely
with browserify. He has been using
https://github.com/jordwalke/npm-react-core as a placeholder, but in
order to keep doing that, it requires another build step and more places
for us to screw up. This just re-uses the NPM module we're already
shipping and is really easy to update.

Note: we may want to pull this out into it's own project eventually, but
for now... move fast?