Remove admin_only property

Because of the problems mentioned in #57 we agreed to remove the `admin_only` property. Now, columns can be excluded using `includes_user/admin` and `excludes_user/admin` for columns on the main model, and using `overrides_user/admin` for columns on child models. The same way you would also have to do it when directly using ColanderAlchemy.

Example:

    register_schema(
        'fouille',
        model.ExcavationPermission,
        excludes_user=['reference_number', 'validated'],
        overrides_user={
            # do not show the 'verified' field of ContactPerson for the user
            'contact_persons': {'excludes': ['verified']}
        })


Closes #57