Fix question answers validation

Hi @lsv,

I've fixed the `QuestionBag::setAnswer()` method (also renamed in plural: setAnswers) because `Certificationy\Certification\Set::setAnswer()` takes too arguments:

1. Question key,
2. Answers values (not keys as it was done previously)

This way, we have correct and wrong answers working properly on your web application :)

Do you think we can refactor this code a little, make unit tests and make it available in http://www.github.com/certificationy/CertificationyBundle ?

Thank for this!