Create a system user and use it for creating users

Since only the system itself creates, updates and deletes user this
commit updates the user model to automatically provide a system user
when saving objects.

It also provides a management command to create a new system user

The default system user is set to '-1', does not belong to any
instances, and has its login disabled.
intermd cmt
Merge pull request #129 from ahinz/topic/system-user

Create a system user and use it for creating users