Description:
    Create a model subclass of parent, used for Single Table Inheritance.

    Both subclass and parent name can be either CamelCased or under_scored.

    This generates a model class in app/models and a unit test in test/unit.

Examples:
    `./script/generate model_subclass admin user`

        creates an Admin model, which will inheritate from User model, test:
            Model:      app/models/admin.rb
            Test:       test/unit/admin_test.rb