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 .
`article.url || article_path(article)` might do it as well, unless you somehow end up with an empty string as the `url`, in which case you'll need that [`presence`](http://api.rubyonrails.org/classes/Object.html#method-i-presence).