Feature/range1d datetime

This will require a bit of discussion, especially around the Date property.

Closes https://github.com/ContinuumIO/bokeh/issues/635
OK sure, it's not a super high priority. I think in the mean time, @kdodia we can just make a `Range1dBase` as well as a `Range1d` function that returns the appropriate subclasses of `Range1dBase`. Then if we figure out the machinery to merge them into a class it should not be much work and the interface will not be impacted.