Can now specify global DateField/DateTimeField validators in settings.py. Two fields are exempt:
./administration/models.py:36:    date = models.DateTimeField(default=datetime.now) # AccessLog model
./sis/models.py:834:    date = models.DateTimeField(auto_now_add=True) # ImportLog model

Closes #105
Merge branch 'master' of https://github.com/burke-software/django-sis