Datepicker today button handler "hack" does not work

During init of datepicker, [these lines](https://github.com/sehmaschine/django-grappelli/blob/master/grappelli/static/grappelli/js/grappelli.js#L68-L71) add functionality that should handle clicks on "today" button by selecting and closing the widget.

Unfortunately this does not work with current selector, due to how jquery ui widget is "refreshed" (where button dom elements are destroyed and created again).

To solve this, I used a [delegated handler](http://learn.jquery.com/events/event-delegation/) like so:

    $(document).on('click', '.ui-datepicker-current', function() {
        $.datepicker._selectDate(grappelli.datepicker_instance);
        grappelli.datepicker_instance = null;
    });

This retains the handler for the generated buttons.