Feature/range1d datetime

This will require a bit of discussion, especially around the Date property.

Closes https://github.com/ContinuumIO/bokeh/issues/635
Recursion depth exceeded is a natural consequence of inheriting `Child.__new__` from `Base`. I think the way to make this work is to override `__new__` whenever `__new__` was used in a super class, however this will require a little more fine tuning to make it work with `HasProps`. I will look into this.