Allow using DatePicker as a controlled input

[marnusw]

There were two problems preventing controlled DatePicker inputs:

1. The underlying TextField threw an error because the DatePicker would always setValue() and this is not allowed on controlled inputs.
2. The date value wasn't formatted when rendering, only upon setting the TextField value.

Both items are fixed here, along with some additional documentation on controlled DatePickers.

[hai-cea]

We use this naming convention to indicate that _isControlled() is private to the component. Since we're passing all the props to the TextField input anyways, you should be able to determine this based on the props being passed in to date picker.

[marnusw]

I've fixed it. I was cheating to avoid duplicating the method, but I'll adhere to the convention strictly from now on. Thanks for the feedback @hai-cea.

[hai-cea]

Thanks @marnusw!