Start on Date classes

NOTE: This pull request is NOT ready to be merged. I'm just looking for feedback on how to proceed at this point.

Would you prefer one big squashed commit? Lots of small commits in a branch? A pull request each step of the way?

How do you feel about the nested classes (like Date::Interval) -- I'm really liking them.

Any and all feedback appreciated. I've been sitting on this for a while, and would like to resurrect it to contribute it.