title: Fullcalendar revertFunc() + dragRevertDuration
body: <p>When manually rejecting an event drop with Fullcalendars 'revertFunc()', it seems that the 'dragRevertDuration' option, as set when constructing the calendar, is not used (contrary to reversions when an event is dragged/dropped on e.g. an other event when overlap = false). Is it possible to use the 'revertFunc()' with the 'dragRevertDuration'?</p>
  
  <p>Fullcalendar Docs (+ dragRevertDuration property)</p>
  
  <pre><code>$('#calendar').fullCalendar({
      events: [
          // events here
      ],
      editable: true,
      dragRevertDuration: 1000,
      eventDrop: function(event, delta, revertFunc) {
          if (!confirm("Are you sure about this change?")) {
              revertFunc();
          }
      }
  });
  </code></pre>