Dont automatically assign admin role.

I'd prefer an admin role is not automatically assigned if there are no admin users already.  There is a rake task that can be used to create an admin or it can be done during installation.

This prevents confusion I ran into when writing extension specs.  I would be calling the user factory to test a feature that normal users should not have access to.  The check_admin callback would end up setting the admin role on the user, and cause my spec to fail.  I end up needing to create 2 users just to test a non admin circumstance without removing this callback.