DateTimeObj class to PHP's OO DateTime

[petsagouris]

Maybe it is time to get the DateTimeObj class to extend PHP's DateTime class so that it becomes more powerful and allow for the elimination some of its code.

Furthermore, the setDefaultTimezone() does not semantically belong into the DateTimeObj object since it tampers with the global PHP configuration for its own good. It should be moved outside that class; maybe into include.utilities.php ?