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 know @mduan suggested to use bson.ObjectId() here instead, which seems like it should accomplish the same thing. @divad12 curious if you have a preference