Vary frequency of questions

[mattwynne]

This is an idea for making the game more interesting - have some questions that are high value, but only appear infrequently. At the moment, all questions appear with roughly the same frequency.

[jhannes]

A simple fix that I've used for this is to just repeat high-frequency questions in QuestionFactory::question_types. E.g. if you have four copies of AdditionQuestion, that will occur four times as frequent as other questions.