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
Hmm… just checking out this method, and it seems odd that we're not calling `flag(this, 'object', components);` to update the context of the assertion. I'm thinking it should be in here.