Added NSDate+Calendar (set of categories on NSDate) podspec

This is the set of categories on [NSDate](https://developer.apple.com/library/mac/documentation/Cocoa/Reference/Foundation/Classes/NSDate_Class/Reference/Reference.html) that allow access to date components such as year, month, week of year, week of month, weekday, day, hour, minute, second in local time zone and many more.