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)

Like mentioned elsewhere, we might want to use "voted" as the verb to describe this action. So, `num_found_useful` => `num_rated_useful`.