Option to start Week period on Sunday and end on Saturday

I do have some code implementing feature request from issue #8, but I am not very happy about it. It adds a static variable $firstWeekday to the Calendar class that one can modify before making any other object or calling a method.

One should add some tests as this is a large change in calendar handling. Sorry about the code drop like this but it may be more useful than continue to keep it for myself.