Add simple fixed quizes.

So far they only support users answering correctly, not wrongly, or using hints.
Points are not counted.
Did not make lettuce/splinter tests yet, since I am not sure I will continue to use those.
Change to use proper unittest suite in tests.py, so test discovery works
better.
Award one point for each correct answer
Display a message if the users answers a question incorrectly
When taking a fixed quiz, users can now skip a question if it is too hard
The point for each question is now divided by two for each attmpt made at that question.
Users can now use hints also in fixed quizzes.

It will however take half a point away from the possible score for that
point.
Made design of fixed quiz closer to training mode
Display a message that the user is in training mode if she has answered at least four questions correctly
Users can now tweet their score, with a link to the quiz they took
Make sure a user can only have one quiz going in her session at a time

This avoids confusion about which points belong to which quiz. It would
be possible to handle many sessions at a time, but this was the simplest
solution.
Add info in the right column about the mode the user is currently in.
Improve Twitter-message when quiz is complete
After a quiz is finished, offer the user to start a new one, or go back to training mode
Fix broken test