Darcy ui fundamental ui element types (issue #18)

I know the RadioButton naming isn't ideal but pushing this up to see if I'm heading in the right direction. I also have been working on darcy-webdriver to add implementations so I'll get that up here soon. Thank you!
I think I told you to extend HasValue but I take it back now. Let's keep this dirt simple (like so) for now. In darcy-web we can model the HtmlDateSelect after the HTML5 spec.

Which makes me wonder: what do you like better, DateInput or DateSelect?

DateInput might actually be more appropriate. Select implies you are limited to a predefined selection, but most date "boxes" allow to also manually type. It also matches HTML (which is both kind of a UI standard and the primary use case for darcy) semantics. But, I could see an argument either way so I'm curious what you think.