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)

It doesn't look like this is robust to repeated votes.

One way to ensure this doesn't get voted on more than once by the same user is to have a field is a list of the user IDs who have voted on this item. Then you make sure you never add repeated elements, and you use the length of the list to get a count.