Made AZ_currentCalendar public.

This is useful when you want the application-global calendar object for general date calculation. For my app, I want to make sure all date calculations are guaranteed to use NSGregorianCalendar. Also, this calendar is thread-safe as well as fast (cached).
1.4.1 (Temporary)

Temporary before my PR is merged into the main repos.

Or perhaps it won't be merged after all :X