Fix the datetime.now default for DateTimeFields
Fix the datetime.now default for DateTimeFields

Passing the function datetime.datetime.now as the default
causes the current time to be used on creation, whereas
previously the time used as a default would always be
the time when the field was defined.

fzadow/CATMAID,fzadow/CATMAID,htem/CATMAID,fzadow/CATMAID,fzadow/CATMAID,htem/CATMAID,htem/CATMAID,htem/CATMAID