Restrict sliders to only top-level ul elements

Currently, markup like the following results in strange and unexpected slider behavior:

```html
<div class="slider">
  <ul>
    <li>
      <ul>
        <li>
          <p>Hi</p>
        </li>
      </ul>
      <p>Hi again</p>
    </li>
  </ul>
</div>
```

Essentially, if your slider contains content with another `<ul>`, an extra sub slider is created. This contradicts the behavior implied in the stylesheet, which scopes the slider to `.slider > ul`. This PR corrects that in JavaScript.

(thx @kyledoherty)