Commenting

Add commenting feature to Hacker News clone. Also formalizes the relationship between `Articles` and `Users` and includes stylistic changes and some basic refactoring. Resolves #12 .
Ah, I see. Yeah, I liked the idea of taking out the instance variable references in the views and replacing them with a method call, but I didn't even notice that would mean I wouldn't need to call that method in the controller now. :+1: