A bunch of fixes :

* Correct the data models to make use of django user management.
  There's still some questions but I think the is_author will be useful to filter author from user
  quickly;
* Finalize the comic data model, looks really cool now;
* Work on the admin interface;

Probably more ... I've forgot.

Signed-off-by: Jérôme Benoit <jerome.benoit@grenouille.com>