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
Is ```-init``` calling ```-initWithTimeIntervalSinceReferenceDate``` by default for ```NSDate``` instances? I guess it should.
If that is not the case you would probably need to override ```-init``` to make sure ```-includesTimeComponent``` is set to ```YES```.
But maybe it