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 .
After our discussion, sounds like you can solve this with something like:

```erb
<%# app/views/shared/_owner_section.html.erb %>
<%= content_tag :div, class: class %>
  Posted by <strong><%= resource.user.try(:name) || 'Anon' %></strong> at <%= resource.created_at %>
<% end %>
```

Usage:

```erb
<%= render partial: 'shared/owner_section', locals: { class: 'posted-by', resource: comment } %>
```