Adding date picker widget

[SylvainCorlay]

Closes #168.

This also removes the old unused eventful containers.

[SylvainCorlay]

Ping @dmadeka @ssunkara1
ipywidgets 6.0dev now has a DatePicker widget. It is a very simple one but more robust than before.

The core ipywidgets Datetime trait type only holds a python datetime with serialization routines. We mimick the python datetime's behavior in terms of timezone awareness.

[MuddyBootsCode]

Does this work for all versions of notebook? I've got v6.0.0rc2 installed and I get:

import ipywidgets as widgets
sorted(widgets.Widget.widget_types)
Out[39]:
['Jupyter.Accordion',
 'Jupyter.BoundedFloatText',
 'Jupyter.BoundedIntText',
 'Jupyter.Box',
 'Jupyter.Button',
 'Jupyter.Checkbox',
 'Jupyter.ColorPicker',
 'Jupyter.Controller',
 'Jupyter.ControllerAxis',
 'Jupyter.ControllerButton',
 'Jupyter.Dropdown',
 'Jupyter.FlexBox',
 'Jupyter.FloatProgress',

DatePicker isn't available as a choice.