make AUTH_USER_MODEL to work with 'app_label.model_name' format
some util functions
Log an entry only if there are changes in model on update
Merge pull request #1 from francofuji/master

Merge pull request #1 from francofuji/master
- Make AUTH_USER_MODEL work with 'app_label.model_name' format
- Some util functions
- Log an entry only if there are changes