Refactor Quiz

Make clean answer a “static method” on the Quiz constructor, since it
was operating algorithmically (i.e. not relying on `this` at all)
anyway.

Merge gradeAnswer into recordAnswer, since grading is part of
recording, and this makes the interface simpler for clients. Also,
create a getPointsForAnswer method, which is the only kind of “grading”
that can really be called such outside of recording.

Set recordAnswer to only unset the currentVerb if the quiz is done;
otherwise, the currentVerb remains as is until nextPrompt is called.
Set up grunt to run qunit tests
In Quiz, encapsulate `remainingVerbs` and `completedVerbs`

Includes creating new `missedVerbs` and `answerIsCorrect` functions
that Controller can use for neater access to information that it
previously got by sniffing into those properties.
`Quiz.remainingVerbs` and `Quiz.completedVerbs` now return a copy of those arrays, rather than a direct reference
Add Sinon for better testing
Preliminary tests for Quiz
Verb bugfix