Backwards compatible Support for Django 1.4 timezone support

Added an import for `django.utils.timezone` for `now()`, and in the case of an `ImportError`, imports `datetime.datetime` as `timezone` instead, for backwards compatible support.

Otherwise, if timezone support is turned on for a model, the modified time is timezone aware, but the `datetime.datetime.now()` is timezone naive, which results in an exception when trying to subtract one from the other.