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
Ignore me, I'm an idiot. This is inside the loop, we are calling `flag(this, 'object', components)` further down in the function… was hidden by GH.