Fixed CalendarUnit constants for iOS

Also added 'minutely.'

These are the constant names as specified in NSCalendar.h, and documented at:

https://developer.apple.com/library/ios/documentation/cocoa/Reference/Foundation/Classes/NSCalendar_Class/Reference/NSCalendar.html#//apple_ref/doc/uid/TP40001451-CJBJEIAF

Thanks for doing all this work!