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 .
Should've definitely noticed that on articles/index, duh. As to in articles/show, I can definitely see where you're coming from. I'm curious to hear @zporter 's thoughts on this since he recommended this pattern to me, though he originally specified using it for `Comments` rather than `Articles`.