React 0.12

- does not change deprecated methods (e.g. merge, copyProperties etc)
- use `object-assign` module to polyfill `Object.assign`

Note that react uses own polyfill for Object.assign - see https://github.com/facebook/react/issues/1998 and https://github.com/facebook/react/pull/2350

Also this will not work on react < 0.12. I think previous versions should have depend on ^0.11.0, not >= 0.11.0.