Drop legacy browser support

No longer supports IE 6/7, Firefox < 4, and Safari < 5.

* Make use of `inherit` to simplify some of the rules.

* Remove a lot of padding and margin normalization, particularly for
  typographic elements, because modern browsers share common base
  styles.

* Add `quotes` normalization. While all target browsers support
  `quotes`, they don't share a common set of quote styles. Opera and IE
  use "curly" quotes whereas other browsers do not. Browsers don't
  appear to set different quotes depending on the language (via the
  `lang` attribute) of the content.

* Remove all list normalizations and they aren't needed anymore.

* Remove a handful of form normalizations that targetted IE 6/7 or
  Firefox 3.