Fix to not define all the FKs/Models

I'm new on your codebase so this PR is more to open a discussion.

I created the scripts/models for my own project, in my case I have a DB with about 80 tables and I want to use sandman only for a subset of my models (not alls), therefore I don't want to have to define all models.

Without this patch I get error like:
    File "/home/areski/projects/Flask/sandman/sandman/model/utils.py", line 69, in prepare_relationships
        other = known_tables[foreign_key['referred_table']]
    KeyError: u'auth_user'