Use rangeOfUnit:inUnit:forDate: to determine the number of weeks in a month

This fixes issue #18 by using <a href="https://developer.apple.com/library/ios/documentation/Cocoa/Reference/Foundation/Classes/NSCalendar_Class/Reference/NSCalendar.html#//apple_ref/doc/uid/TP40001451-BBCDBDDG">`-[NSCalendar rangeOfUnit:inUnit:forDate:]`</a> to determine the number of weeks instead of examining the week number of particular weeks, which failed for UK dates (which can have a week number of 53)