PR for _sass branch

This branch is now ready for prime time. Thanks to all involved:
@gregrickaby @bradp @hugobaeta @obenland @sabreuse @MichaelArestad 
(and anyone else involved - you rock, thanks!)

Relates to: #548 
```scss
$default-padding: 1.5em !default;

ul, ol {
  margin: 0 0 $default-padding $default-padding*2;

  li {
    > ul,
    > ol {
      margin-bottom: 0;
      margin-left: $default-padding;
    }
  }
}

ul {
  list-style: disc;
}

ol {
  list-style: decimal;
}