withState() callback

Adds a second parameter to withState()'s state updating function — a callback that will be executed once the update is completed and the component is re-rendered. This simply exposes the feature by React's setState(). https://facebook.github.io/react/docs/component-api.html#setstate

Also fixes an issue where defaultProps() did not override props with values of undefined.

Thanks @istarkov for the PRs #15 #14