Added 'addDocblock' option which will add the /** @jsx React.DOM */ comment to the source when it's missing like the 'reactify' module does

After having used [reactify](https://github.com/andreypopp/reactify) to build my front-end components, I ran into problems with using my components in Node. After investigating the difference in compilation between the  two modules it seemed that the problem is the @jsx instructions missing, which reactify adds automatically when missing.

Instead of updating all my files, I thought this wouldn't be bad optional behaviour for this package either.