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
Please maintain the existing indentation style. This should be written as follows:

```javascript
return React.createElement(
  'div',
  { className: 'abc testing-class' },
  'separator text',
  React.createElement('span', { className: 'my-class other-class cool' }, 'my other span text'),
  childComponent({}),
  childComponent({ myVar: 5 })
);
```