Added a calendar widget (#11).
Added leading (previous month) and trailing (next month) days to the calendar.
Borrowed HTML from standard calendar widget.
Added navigation.
Moved WPT_Calendar_Widget class to same file as WPT_Calendar class.
Added [wpt_calendar] shortcode.
Removed option to use productions on dedicated listing page.
Added test for calendar shortcode.
Fixed CSS for calendar navigation.
Closes #11.