Displays a calendar for date picking.
Shows the calendar. It will appear at [x, y]. If date is specified, it will be pre-selected. All dates are in form of [year, month, day], starting from 1. When user selects a date, a callback(date) will be called.
Further customizations can be done by setting some properties:
Date::getDay()
returns 0, i.e. Sunday.
Calendar.dayNames = ["Mon","Tue","Wed","Thu","Fri","Sat","Sun"];
Calendar.monthNames = ["Jan","Feb","Mar","Apr","May","Jun","Jul","Aug","Sep","Oct","Nov","Dec"];
Calendar.specialDays = [0,0,0,0,0,1,1];
Calendar.dayZeroIndex = 6;
Calendar.weekStartIndex = 6;
Calendar.show(100, 100, function(){});