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
Also, I'm wondering if they might have converged on a single interface for text components maybe? I know previously that it was possible for a DOM component to just have `props.children` set as a string, instead of a proper text component. Wondering if that scenario still exists.