I actually have a weird issue with using add_question over their. 
This question only needs to be added for twitter, but not ussd.
This means that the ``is_complete()`` will no longer work for ussd since it is never answered.

 Also, once they have completed the questions - they have technically "completed the quiz" despite not having put their number in, so I guess it's okay that the metric still fire's here? Not ideal :/ 

An idea would be to extend from answerwinquiz and instantiate the answerwin.twitter.quiz if twitter is the delivery class. But I don't want to over engineer at this stage. 

Your thoughts?