Ratings and Reviews for Add-Ons

Still a work in progress but this PR will eventually add Reviews and Ratings to the details page.

We have a number of things to think about such as what user system should we use (if any)? How do we let users write the review. How long/short can a review be? Do we allow HTML, images ect. How do we moderate comments.

@da-anda, @MartijnKaijser and @natethomas I would be interested in your opinions.

Example here 
![reviews](https://f.cloud.github.com/assets/1050512/1562546/7c898518-504a-11e3-8532-5ef1ff2083de.png)