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)

Yeah, I'll definitely change that so we don't use `user_course_id` to determine if a user can vote or not. 

In terms of server side vs client side, I found it to be easier to implement and cleaner to do the comparisons on the server, rather than with Backbone. But I'm definitely open to shifting the logic, if you think it'll increase performance and/or readability. Though ideally I'd take something that's easier to understand as long as the performance difference is fairly minimal. 

What do you think @mduan?