Date improvements

Added PKDate class to represent dates without a time. There are two basic principles:

* If the date contains a time component, it acts as a normal NSDate
* If the date does NOT contain a time component, its date represents the day component in UTC time
Pretty sure it is, but I should double check. Apple says for subclasses you need to override `-initWithTimeIntervalSinceReferenceDate:` and does not mention `-init`.

Source: https://developer.apple.com/library/mac/documentation/Cocoa/Reference/Foundation/Classes/NSDate_Class/Reference/Reference.html