Adding datetime type react component

This `datetime` component is functionally equivalent to its "pre-react" counterpart.

TODO:
* client-side validation (per @JedWatson, will be implemented post-react)
* strict parsing with `moment` (will submit separate PR for both this and the `date` component)
* optional dropdowns for time input using `select` component (waiting on completion of `select` component)
* optional number-steppers for time input (if we still think it's a good idea).