Add support for custom calendars

This datepicker has good support for localization, but for some, this ain't enough. I need the support for custom calendar too. I need to use this datepicker with Jalali Canlendar (instead of the default Georgian Calendar).

In my own case, I built a new class called JalaliDate with the same methods as Date class (the default javascript Date class) such as setDate, setFullYear, getDate, getMonth, etc. then in the code, I replaced all "Date()" references with "JalaliDate()" and it worked just perfect in my case.

I think it's a good idea to add the support for this type of custom Date classes. so that when localizing the calendar, one would be able to set the "Date" inteface too.

PS: if you think it's needed, I can provide you with my JalaliDate class which I talked about.