title: Fullcalendar eventOverlap issue
body: <p>I use fullCalendar 2.6 and I only want one allday event per day.
  I use this : </p>
  
  <pre><code>eventOverlap: false
  </code></pre>
  
  <p>It works if I put an event and move it on a day which has already an event : they won't overlap.
  But if I click on a day which has already an event, it overlaps and I get 2 events (or more) at the same date... 
  I use this too on my select function : </p>
  
  <pre><code>overlap: false
  </code></pre>
  
  <p>which does not do the trick.. </p>
  
  <p>What can I do ? any idea ?</p>
  
  <p>And another issue is when I use ajax to send start/end dates, it only works when I click to add an event but not when I move it using arrows when I put my cursor on the edge of the event to change its size... </p>
  
  <p>My code : </p>
  
  <pre><code>    function renderCalendar() {
          $('#calendar').fullCalendar({
              header: {
                  left: 'prev,next today',
                  center: 'title',
                  right: 'month,basicWeek,basicDay',
              },
              defaultDate: dateNow,
              defaultView: 'month',
              lang: 'fr',
              height: 'auto',
              editable: true,
              allDaySlot: true,
              weekNumbers: false,
              timeFormat: 'H:mm',
              slotEventOverlap: false,
              weekends: true,
              selectable: true,
              selectHelper: true,
              eventOverlap: false,
              select: function(start, end) {
                  var title = 'Occupé';
                  var eventData;
                  if (title) {
                      eventData = {
                          title: title,
                          start: start,
                          end: end,
                          allDay: true,
                          overlap: false,
                          color: '#bf0000',
                          textColor: '#ffffff',
                      };
                      $('#calendar').fullCalendar('renderEvent', eventData, true);
                  }
                  $('#calendar').fullCalendar('unselect');
                  alert(start);
                  alert(end);
              }
          });
  </code></pre>
  
  <p>Thanks for your help !</p>