Support timezone in all date/time base classes
Support timezone in all date/time base classes

First step to using datetime for dates; more to come.

jonfoster/pyxb2,jonfoster/pyxb2,pabigot/pyxb,balanced/PyXB,CantemoInternal/pyxb,jonfoster/pyxb1,CantemoInternal/pyxb,jonfoster/pyxb2,pabigot/pyxb,balanced/PyXB,jonfoster/pyxb-upstream-mirror,jonfoster/pyxb-upstream-mirror,balanced/PyXB,jonfoster/pyxb1,jonfoster/pyxb-upstream-mirror,CantemoInternal/pyxb