Also write up section on number inputs

This moves the current fixture architecture to one based around create-react-app. There are a few important things to note:

* The react-loader.js script is always loaded before the bundle and will populate React and ReactDOM on the window. It is then read from the window by all components.
* The UI for the "React Sandbox" or "React Fixtures App" is also rendered with whatever version of React the user has selected. This means we dont have to deal with iframes or worry about multiple versions of React potentially interferring. But it also means that all components must be written using createClass to be fully backwards compatable. I tested back to 0.13.1 and it works fine.

Webpack requires absolute paths here