Feature/range1d datetime (WIP)

This will require a bit of discussion, especially around the Date property.

Closes https://github.com/ContinuumIO/bokeh/issues/635
It's probably possibly to make this work with `Range1d` as both a base and a factory but we'd have to go down the metaclass or class decorator route. I"m not necessarily opposed to that but we should document, comment and explain such a thing very well.