Implement basic multiple choice question parsing.

Right now, just the question text and choices are parsed. Choices
consist of the ID assigned to it, the choice text, and whether it is the
correct choice or not.
Move CalculatedQuestion methods out from Question.

Since we started with only calculated questions, it made sense to have
only 1 question class. But now that we have multiple question types,
there needs to be a separate question classes, so now we have
CalculatedQuestion and MultiChoiceQuestion classes.

This commit just moves the Calculated Question specific methods and
variables to the CalculatedQuestion class.