Feature/range1d datetime (WIP)

This will require a bit of discussion, especially around the Date property.

Closes https://github.com/ContinuumIO/bokeh/issues/635
This asks for trouble. There are three classes of exceptions that this will catch: 1) system/special exceptions that you must not catch (`SystemExit`, `KeyboardInterrupt`, maybe other), 2) exceptions you don't want to catch (e.g. `MemoryOverflow`, etc.) and 3) exception (preferably) or exceptions that you want to catch. At minimum you should add a special case for 1). At best you should catch specific exception(s) or remove this block at all. I think what you should do is to catch `ValueError` which stands for validation error. Note this will be soon renamed to something more appropriate and unique (e.g. `ValidationError` or `TypeCheckingError`). To expand a little more, suppose you also catch `TypeError` here, then you may make debugging for yourself and/or others harder, because Python raises `TypeError` is all sorts of situations that are not related to the logic of this program, but can indicate trivial implementation errors (not enough arguments to a function, etc.). As there is no compilation phase and thus any static verification, lets make sure that our programming style doesn't make development even harder.