React 0.12

- Replace ReactDescriptor with ReactElement (fix #433)
- Replace removed `mixInto` with Object.assign - using `object-assign` polyfill
- Other deprecated methods are not replaced (e.g. merge, copyProperties etc). probably better to fix them together.

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.