update jquery.impromptu
reveal does not remove marks; show correct answer next to student's

The student does not lose their marks when they press Reveal. cf #233
The correct answer is shown next to the student's. cf #88
Still need to do a correct-answer display for MCQ parts.

New localisation string: 'part.correct answer'.

Finally, we're getting to the reason I started the knockout rewrite!
Next step: review mode.