Mtk multichoice

Trello Card: https://trello.com/c/CK5rp02q/20-as-a-user-i-can-create-multiple-choice-questions-answer-options

![multi_choice](https://cloud.githubusercontent.com/assets/4560985/5012651/b33fad78-6a55-11e4-842c-49826fe84d01.gif)

`.where(....).first` can always be simplified to `.find_by(....)`