Export `window.React` so that the react-devtools can find it.

The chrome tools first looks to see if `React` is defined globally, then if it's available via a call to `require()`. See https://github.com/facebook/react-devtools/blob/master/injected/ReactHost.js#L42-L48
Merge pull request #17 from ryan953/patch-1

Export `window.React` so that the react-devtools can find it.