Add ability to inject custom calendar with backward compatibility

[nitinkraghuwanshi]

Issue :
I needed to update few calendar's property e.g. firstWeekday, because JTDateHelper was returning non mutable calendar object , i was not able to do so.

Solution proposing
As JTDateHelper is having three instance variable , NSCalendar, NSLocale and NSTimeZone, but only 2 are available to inject.
There can always be a need to update some properties of NSCalendar and hence I added ability to inject full calendar object itself with maintaining the backward compatibility of current code.