Users can rate and unrate recipes

[Trello Card - Users can rate and unrate recipes](https://trello.com/c/Za8WSgS6/22-users-can-rate-recipes)
![screen shot 2014-11-20 at 5 24 21 pm](https://cloud.githubusercontent.com/assets/2309768/5134231/27b312f0-70da-11e4-8d8f-0f7a481d6ac9.png)
![screen shot 2014-11-20 at 5 24 37 pm](https://cloud.githubusercontent.com/assets/2309768/5134232/27b6f910-70da-11e4-9636-9b4645189154.png)


This is very inefficient. How about using `ratings.sum(:value)`, which should issue a SQL `SUM` command: http://api.rubyonrails.org/classes/ActiveRecord/Calculations.html#method-i-sum