Introduced first implementation of onChange handler

@markijbema this doesn't feel right, since there is no tight coupling between the `this.state.selected` and calling the `this.props.onChange` handler. The general consensus seems to be that you need to pass the selected as a prop instead of a state, what do you think?

I'm also not so sure about passing a moment.js object in the handler, since I'm planning on removing the moment dependency as soon as we can (10+ kb is just a little too much).

Fixes #12