db schema and versioning

A number of issues have mentioned a need for some work on the database schema.
Currently results are not stored (they are recalculated every time its needed),
and some of the data is stored in a serialized format, making writing efficient
queries impossible.

Specifically, this makes view integration
difficult if not impossible. Storing more regular results could not only allow views,
but all sorts of reporting possibilities.

Additionally, to make the module more suitable for academic applications, using
drupal's node revisioning system could really help out. It would be totally optional
to use, but allow both questions and quizzes to be revisioned. So for example a quiz
is created with some questions, and some people take it and get results. Then an
admin edits a question, or adds a new question, and now its (optionally) a new version
of the quiz. So results from the original quiz are not equivalent to results from the new revision.

As long as there were update scripts to move from the old schema to the new schema,
the versioning system is pretty transparent to the user, and uses the standard drupal system.

I plan on having a patch available this week for folks to evaluate, so please let me know
if there are other related features that would be a good idea.