Feature quiz

So I added a way to take a quiz. I'm not sure if it is how you guys would have done it, so suggestions/improvements are welcome. 

Basically now, there is all a "Install Quiz" link in the course admin page, that takes the user to a quiz generator page. The quiz currently has to be multiple choice with only one answer, and everything has to be text. I also added 2 columns to the assessments table in the db, quiz and quizData. The column quiz is a boolean and the quizData is a json string for the actual quiz. Then, when you click on the assessment, if it is a quiz, instead of a handin button, it's a take the quiz button. This takes the user to the quiz page and after it is taken, to the handin history page. By default, a quiz can only be taken once. 
Also, if you use text, you may want to add a default_scope that excludes it, so that the database doesn't need to retrieve the entire blob every time it gets an assessment out of the DB.