new: overridablility of formatDates functions

As a developer I need to be able to override ``formatDate`` or ``formatDates`` by a custom function from outside fullcalendar, and I cannot find a way to do it. I need to do that because I need to get proper localization support from old [DateJS](http://code.google.com/p/datejs/wiki/FormatSpecifiers), [moment](http://momentjs.com), or even the recent [toLocaleString](https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Global_Objects/Date/toLocaleString) coming into javascript. 


That's what is possible with my patch:

          $.fullCalendar.formatDate = function (date, format, options) {
                // ... my custom function
          }

or:

          $.fullCalendar.formatDates = function (date1, date2, format, options) {
              // ... my custom function
          };

The patch is very small, and I've provided a test/demo file.

Any comments are welcome.