Adding some rudimentary scoring into the Quiz / Question models
* A Quiz score is 0 if incomplete, else is the score / question count
* A Question is "correct" if complete and
    (not answerable or
     has no correct answers or
     the user has selected the correct answers)
score: deal with a quiz w/no questions
Merge pull request #10 from ccnmtl/0.9.4

Adding some rudimentary scoring into the Quiz / Question models