Revert "Revert "Country questions"" [MERGE FIRST]

Reverts alphagov/smartdown#93

Sorry for the git mess.
Can we also please include in this patch validation of country questions?? What happens if the value entered is not a country. I would expect an error in the style of multiple choice like "Invalid country" etc. 
This also means that we wouldn't run the risk of returning ```nil``` as the return of this ```humanize``` method.