Upgrade to React 0.12.0

In this PR:

- Upgrading to react v0.12.0.
- Adding the ability to lookup rendered components by tag (not DOM or text components)
- Adding ability to check if component is a valid react class
- Adding ability to check if element is a valid react element
Did the interface for text components change? It seems unlikely that there would be three difference interfaces for text components – I would imagine if it changed that one of the previous interfaces would no longer exist.