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 took a stab at some refactoring.

First, I used the [`cycle`](http://api.rubyonrails.org/classes/ActionView/Helpers/TextHelper.html#method-i-cycle) helper method to toggle the classes:

```erb
<div class="comment <%= cycle('odd-comment', 'even-comment') %>">
```

Then, I used some jQuery selectors to find the right divs and buttons:

```js
$('.comment').each(function() {
  var $this     = $(this),
      $replyDiv = $this.children(".reply-div:first"),
      $formDiv  = $this.children(".reply-form:first"),
      $buttons  = $this.find(".reply-button:first, .cancel-button:first");

  $buttons.click(function(e) {
    e.preventDefault();

    $formDiv.toggle();
    $replyDiv.toggle();
  });
});
```

I hope this makes sense.  Maybe you can run some of this by a FED sometime and get their expertise.