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. In that case, I think having a `url_host` instance method on `Article` would be a better approach. That way you could memoize this without needing to convert the `url` twice.  Thoughts?

```ruby
class Article < ActiveRecord::Base
  def url_host
    @url_host ||= URI(url).host
  end
end
```