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 .
Definitely. Question on that note: the only real difference between displaying the article info on `index` versus `show` is that I give some `div`s a slightly different class name to make some stylistic changes. Is there any way I could have some kind of partial that would just make the `div`s with the proper information and then apply style to them differently? Like would it be worth having a different CSS file included in `index.html.erb` and 'show.html.erb`?