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 .
Should probably memoize a couple of variables here:

```js
var $this   = $(this),
    $parent = $this.parent();

if ($parent.hasClass('odd-comment')) {
  $this.addClass('even-comment');
} else {
  $this.addClass('odd-comment');
}
```