title: Listener for "date change" in FullCalendar?
body: <p>Is there any way to attach a listener to FullCalendar that will be fired whenever the currently viewed date is changed, i.e. if you are in Month view, going to the next month would fire the event passing the Date for the first day of the month, and the same for changing weeks in Week view, day in Day view etc.</p>
  
  <p>The only exposed listener that would be close to this is the dayClick callback, but that's no good because it only fires when a user clicks in an actual day's cell, not for example when the Next/prev/today controls are pressed in the header bar.</p>
  
  <p>I want this functionality to be able to sync the currently viewed month in FullCalendar with a ExtJS datepicker component, so if the viewed month is changed in FullCalendar, then the Datepicker is updated to show that same month. (think Google Calendar's UI with its "mini calendar" in the top left being the datepicker).</p>