Allow voting on reviews

First draft, posting a PR mainly since I have a few questions, and for some early feedback. 
![rate_reviews_1](https://cloud.githubusercontent.com/assets/2644780/3867707/5265ae9c-200b-11e4-83eb-ac71a6d84bc3.png)

I think it's somewhat inconsistent to use a `user_course_id` field to indicate if the user has voted on a review. Instead, all reviews should be given `user_course_id` and you should use another field `voted` to indicate if the current user has voted on the review.

However, I don't think it's necessary to do any of this server-side. We've mostly been keeping the models that we pass to the client-side pretty close to their representations in the DB and doing this kind of logic on the client. To decide if you should render the voting buttons (based on whether the user has already reviewed the course) you can pass the reviews a user has voted on to the Backbone User model. Then for each review check if the user has voted on that course before you render it. You also shouldn't need to store `user_course_id` or `review_type` on the objects server-side as you should be able to do this on the client.