Add basic running test

Close #42

- Test for 'off' as id input
- Test for login

    Uses `instance_double('User')`, stubs for `User.find_by(username: 'testid')`, `User#try(:authenticate, 'testpw')` and `User#admin?`.
- Add `only_have_processors` matcher

    ``` ruby
    expect(cached_processors).to only_have_processors(%i(
      login_menu goodbye_menu
    ))
    ```
    expects
    ``` ruby
    cached_processors.size == 2 &&
      cached_processors[:login_menu].is_a?(LoginMenu) &&
      cached_processors[:goodbye_menu].is_a?(GoodbyeMenu)
    ```
    to be true.