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 .
I'm wondering if this could be:

```ruby
def article
  @article ||= if params[:id]
    Article.find(params[:id])
  else
    current_user.articles.build(article_params)
  end
end
```