[REF] get rid of the mx.Datetime C dependency, meaning better portability,...

[REF] get rid of the mx.Datetime C dependency, meaning better portability, especially to alternative Python interpreters, better maintainability; implements this blueprint: https://blueprints.launchpad.net/openobject-server/+spec/get-rid-of-mx-datetime-dependency; largely the work of Cristian Salamea (GnuThink)

bzr revid: rvalyi@gmail.com-20091213181601-4k4mhtgnv2qynrjo