Feature/range1d datetime (WIP)

This will require a bit of discussion, especially around the Date property.

Closes https://github.com/ContinuumIO/bokeh/issues/635
> numpy is a hard requirement no need to put this in a try block

Is this really true? I thought we will allow using core bokeh without such dependencies (e.g. serializer will work regardless of installed dependencies, unless this was change/broken recently). If we want bokeh to run in other interpreters than cpython, then we should pay attention to this.