Release 0.12.2

I figured we might as well get this done.

- [x] bump version to `-pre`
- [x] Cherry pick everything from https://github.com/facebook/react/issues?q=milestone%3A0.12.2+is%3Aclosed
- [x] also #2726
- [x] Update docs for new attributes
- [x] Update Changelog
- [x] Blog post
- [x] generate starter pack
- [x] Update release date if we don't do this today