Renamed +defaultCalendar to +defaultCalendarIdentifier, and added corres...

I noticed your performance note about creating NSCalendar objects all over the place, and saw a simple way to fix it by using an implicit calendar instance based on the defaultCalendarIdentieifer. Now it will create a single instance at startup, and another if anyone ever calls +setDefaultCalendarIdentifier. I also renamed +defaultCalendar to +defaultCalendarIdentifier, since that seemed more accurate.

I didn't add any new tests, but the existing test suite still passes.