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
So it depends on what we want to do with that chai helper method. If we want to be returning DOM components or composite components. To be honest we probably need two methods one for each.

I initially started with `TestUtils.scryRenderedDOMComponentsWithTag` however that returns a DOM component which, when you go to inspect the props it doesn't have the same props as a composite component. I was trying to keep more or less the same functionality as the previous specs had outlined.

Another thing to note is that you no longer have access to state on a DOM component either.