* Update `create_user` to set last_login with a default.
**Note: this change has been done for the upcoming version django > 1.7.0.**
`User.last_login` default is removed from django > 1.7.0. For existing
project using `django-user-management` project migrations would be run
after `django.contrib.auth` migrations. The project migrations will cancel
`last_login` `IS NULL`.