Set up more tasks for rake testing

Make the test task depend on test:unit and test:acceptance, so we test in the
quickest way possible.
fix up mqcd -> mcqd typo
Add initial acceptance tests for test / answer_key

The notion is that /test/1 will contain a set of questions and answers, and
that /test/1/1.txt would be the first scrambled test, and /answer_key/1/1.txt
is the answer key for it.