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
It would probably be best to have a more explicit comparison:

```javscript
if (TestUtils.isTextComponent(textComponents[i]) && textComponents[i].props === text
    || !TestUtils.isTextComponent(textComponents[i]) && textComponents[i].props.children === text) {
  // ...
}
```