Replace excludes_user/includes_user with admin_only property

So far, to hide a field in the user form, you had to write the following:

    register_schema('tests_persons', Person, excludes_user=['validated'])

This PR introduces a `admin_only` property which can be directly set in the `colanderalchemy` configuration of the column:

    class Person(Base):
            __tablename__ = 'tests_persons'
            ...
            validated = Column(Boolean, info={
                'colanderalchemy': {
                    'title': 'Validation',
                    'label': 'Validated',
                    'admin_only': True
                }})

    register_schema('tests_persons', Person)

With this change you can no longer hide fields in the admin form (using `excludes_admin` or `includes_admin`), but I think this is not required.

This PR also builds upon #18, so please merge #18 first.