Feature/range1d datetime (WIP)

This will require a bit of discussion, especially around the Date property.

Closes https://github.com/ContinuumIO/bokeh/issues/635
I'm not sure if this override is really necessary, but if so, then it shouldn't raise any exceptions. It's `validate()`'s job to verify its input and complain if necessary, and `transform()` assumes that `value` is OK, but it will convert it to a more useful, possibly canonical, value.