jsx whitespace codemod utility

Adds a new CLI tool that will shim whitespace for a JSX file written for React 0.8 (or before) so that it is roughly functionally the same with the changes to the JSX whitespace rules made in React 0.9.

There are a few minor caveats to the way this works that you should be mindful of when using it:
* Whitespace shims are added by inserting a `{' '}` where there was implicit whitespace before.
* Adding a `{' '}` to a JSX expression means a new `<span> </span>` will be rendered to compensate for that whitespace.
* Adding a `{' '`} in to a JSX expression that previously only had a single text node child will make `this.props.children` for that component switch to an array of strings rather than a single string representing the text node as it was before