Add GDateTime to GLib

GDateTime is an opaque data type containing a date and time
representation. It's immutable once created and reference
counted.

https://bugzilla.gnome.org/show_bug.cgi?id=50076



Based on the code by: Christian Hergert <chris@dronelabs.com>

Signed-off-by: Emmanuele Bassi <ebassi@linux.intel.com>