Count answered questions.
Include score from last ten questions.
Change 'answered_correctly' to 'given_answer' in db.

This seems cleaner. Partly because it fits better with the existing
code, but we can always have a method for the one that isn't directly in
the database. But it makes potential SQL queries nicer:
`answered_correctly` is just `given_answer = correct_index`, but
`given_answer` is slightly more complicated in terms of
`answered_correctly` and `correct_index`.
Report three kinds of score in a table.

Needs CSS.
Importing credence questions requires argument.

Also removed an out of date comment.
Add num_answered column to CredenceGame.

Calculating it by listing all questions and checking whether they've
been answered is silly.
Add some tests for CredenceGame.
Add credence-todo.org.