Added events on show and hide of Calendars

Added events when showing and hiding the calendars. Use case : I added a range "forever" which set start and end to empty values. If user opens the picker again, and select "custom range", I want to set start and end to today. For example:

```
$('#datepicker').on('showCalendar.daterangepicker', function(event, picker) {
    var start = picker.startDate,
        end = picker.endDate;

    if(!start.isValid() && !end.isValid()) {
        $widget.data('daterangepicker').setEndDate(new Date());
        $widget.data('daterangepicker').setStartDate(new Date());
    }
});
```