quiz_take_quiz should be able to give some subset of a quiz, rather than just "a question"

quiz_take_quiz() is holding us back from adding some very cool features. Most notably, we cannot organize a quiz in any other way than as a serial progression of questions, where each question comes after another question one at a time.

Sivaji has done an admirable job working around this for AJAX quiz, but for server-driven quizzes we are still prevented from doing things like:

- displaying an entire quiz on one screen
- grouping questions into groups and displaying a group of questions

While it doesn't need to be addressed for 4.0, we should definitely begin discussing and designing an alternative method for giving quizzes that does not enforce the 1-question-per-page rule.

This will involve a major refactor of quiz_take_quiz(), which stinks (that function is huge and icky already), but this prevents a great opportunity to re-do the biggest and ickiest part of quiz.

So... thoughts?