Add table for users and migrate schema.
Add new controller with relative model for the user authentication.
No sign up will be implemented, users will be added directly to the DB.

Move nonlogged layout to default (will be shared among logged/nonlogged), create navbar.
Fix layout '_default' to really be the default one for the whole app.
Fix navbar with more correct links.
Create DbMaint model and controller.
Add methods to session helper.
Implement correctly the authenticate method.
Fix wrong redirect.