Cmckee/merit

Think I finally fixed schema problems.
I don't like this and don't think it's going to work properly, but not sure how to fix it right now.

* I would probably extend the since_date to be 3.months.ago to encompass overall leader for the entire duration of the course. This would be better if we could tie it to the duration field set for the course of that cohort.
* Based on this targeting the :users table generically like that and on the query below, I think this is going to build a leader board off of all of the users. Not just limit it to only the users for each of the cohorts. So if you have two courses with cohorts going at the same time, the leader board would be built off of users across both courses. Again, not sure how to fix at the moment. Just something to think about.