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)

Is the purpose of this to add a `user_course_id` to `course_review` and `professor_review` dicts? If so, this way of doing it is not very clear. You should be more explicit about what this is doing.