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
Just curious here, I thought skipping ```NSHourCalendarUnit```, ```NSMinuteCalendarUnit``` and ```NSSecondCalendarUnit``` in the ```-components:fromDate:``` call would be the same as setting ```hour```, ```minute``` and ```second``` to zero. Is the behaviour different?