Twenty Thirteen: minor style additions to calendar widget

[MikeHansenMe]

With the look that Twenty Thirteen is going for I think we can improve the hover state or interactions with the user. This could be improved in other widgets as well but I think we can start with calendar.

Improve hover effect for the date "tiles" so that the whole tile interacts not just the text link.

See attached patch for example.

[MikeHansenMe]

minor css changes

[lancewillett]

This is a nice usability enhancement, thanks.

Patch notes:

• Per CSS style guide we add a newline between blocks
• The padding property on .widget_calendar a is now extraneous and can be removed

[lancewillett]

In 23525:

Twenty Thirteen: usability improvement for calendar widget links. Props MikeHansenMe, fixes #23606.