Example demonstrating how to use React with RequireJS

The good news is that `build/react.js` works great with RequireJS.

I decided to discourage in-browser transformation, since `JSXTransformer.js` doesn't interoperate nicely with the RequireJS version of `require`, and I'm still struggling to see any benefit in making it do so.

The new `bin/amd` script makes precompilation pretty straightforward, I think.

cc @zpao @petehunt 

Closes #28.