Feature/range1d datetime (WIP)

This will require a bit of discussion, especially around the Date property.

Closes https://github.com/ContinuumIO/bokeh/issues/635
@mattpap, according to @bryevdv it was yourself who added this property, and based on our discussion yesterday he suggested I reach out to you for more information.

We're trying to get Range1d to accept `datetime.datetime`, `datetime.date`, `pandas.tslib.Timestamp`, and `numpy.datetime64` types. So there are a few options here; I could:

1. Add a separate Datetime property alongside Date (i.e. leave your code alone)
2. Refactor Date into a more selective property (make it do what we want it to do)
3. Add some more verification and transformation code to Date (merge and combine functionality)

Do you have any suggestions or preferences?