Ensure audit_date field is timezone-aware if USE_TZ is true

Hi,
Django > 1.4 uses the USE_TZ setting to determine if dates and datetimes are timezone aware.  This causes a warning when using audit log as the audit_date field isn't timezone aware.  This edit fixes that.  Also, apologies for the whitespace cleanup, its the result of a setting in my IDE.  If this is a problem, let me know and I'll issue another request with only the code changes.