Click event callbacks on table day headers in all views. The callback is called headerClick() and can be set using the option headerClick: function(){}.
Week number headers got no callbacks. No default behaviour is defined. If the headerClick option is not set, the event is ignored.
All day headers (TH elements) in basicDay, basicWeek, agendaDay and agendaWeek views now have data-date attributes, similar to the day TDs in month View.
Headers in month view got no data-date attribute because it doesn't make sense here.
The headerClick callback has the following attributes:
  ev: Native JS event object
  view: FullCalendar View object
Added test case for table header callback. User interaction is required, instructions on test page.