Add initial support for drag and drop of external events/elements
Make the external event selector configurable.

Also make drag&drop and external drop support configurable.
Fix drag2resize. Don't redraw the calendar when resizing events.
Some refactoring to clean up and/or remove duplicate code
Bugfix.

Fullcalendar doesn't include the last day in the event, however we do.
To be consistent we need to remove a day from allDay events after they have
been dragged and dropped.
Bugfix. Check that the calendar control has an "id" attr