Expose user_session in method `token`
Allow re-use session and re-logging in

Specify Hash with key :email, :password, and :token to re-use session
and re-logging in if required.

    nico = Niconico.new(email: 'xxx@xxx', password: 'secret',
                        token: 'user_session_***')
    nico.login

    p nico.token