Add propTypes to improve reusability

`propTypes` are [recommended](http://facebook.github.io/react/docs/reusable-components.html). They improve reusability by defining how we should use a component.

I couldn't find an automated way to publish this to the `gh-pages` branch. Maybe that is another thing we should add?

Thank you for the simple yet great project, @jugoncalves!
Just fixed it in 1edb213 :)