Use webpack to make browser builds

Run `webpack` to try it. Things to fix:

- [ ] grunt integration
- [ ] banner should have "React with Add-Ons" or "JSXTransformer", not always "React"

```
   raw     gz Compared to last run
     =      = build/JSXTransformer-previous.js
 -4852  -4089 build/JSXTransformer.js
     =      = build/react-previous.min.js
     =      = build/react-test.js
     =      = build/react-webpack.js
 +5743   -625 build/react-with-addons.js
-19304  -3636 build/react-with-addons.min.js
 +5619   -149 build/react.js
-17906  -3384 build/react.min.js
```