Feature/range1d datetime

This will require a bit of discussion, especially around the Date property.

Closes https://github.com/ContinuumIO/bokeh/issues/635
> `# Range1d([0,10))`

If only we could have this syntax ;) But speaking seriously, it's a little disappointing that in Python we have to have a built-in function to create ranges and we have only one type (non-inclusive). I wonder if anyone considered making `2..3` equivalent to `2.__dotdot__(3)` or if so, what are the issues. Well there is at least one, mainly `2.` being a valid float literal. Getting rid of this syntax would also make Python more regular:
```python
In [1]: a = 1

In [2]: a.bit_length()
Out[2]: 1

In [3]: 1.bit_length()
  File "<ipython-input-3-069f7d2f7d97>", line 1
    1.bit_length()
               ^
SyntaxError: invalid syntax
````
Unfortunately a good chance to do this was missed (py3).