Bump to version 0.4

Changes since 0.3:
* Added a WSGI-based HTML interface.
* Added user keys to answer database and stack (for multi-user support
  in the WSGI interface).
* Added Question.accept_all for open-ended questions.
* Extended ChoiceQuestion to support multiple choice questions
  (display_choices) which default to "select one of ...", but can be
  configured to "select all that apply" (multiple_answer).
* Added a new software-carpentry-pre-assessment quiz.
* Assorted minor fixups.

I bumped any existing quizzes that use new features to v0.4, but left
the others alone.