'networkTime' method now returns NIL if the timeIntervalSinceDeviceTime ...

...has not been calculated yet

I needed a way to tell if the date retrieved by networkTime is the one really from the network or just a "fake".
In this way if the NetworkClock is not ready yet, it will return a nil value indicating that id does not have a valid value yet (it's on the user to choose if wait for a valid value or just use the regular system date).