Authentication

Authentication/authorization backends belong in haas/auth_mechanisms .  It's
possible that the API of that plugin will have to be expanded to cover what
the CLI needs to do.


WIP
Please use SQLAlchemy's [count()](http://docs.sqlalchemy.org/en/rel_0_9/orm/query.html?highlight=one#sqlalchemy.orm.query.Query.count) or [exists()](http://docs.sqlalchemy.org/en/rel_0_9/orm/query.html?highlight=exists#sqlalchemy.orm.query.Query.exists)