Remove `hr` and separate some margin normalizations

Recommend that people supporting IE6/7 do not use the `hr` element
at all. It requires far more work than just normalizing margins.
IE6/7 do not collapse margins set on `hr` with margins of pre- or
proceeding elements.

Separate the margin normalizations for `p` and `pre` from that for
`h3`. Despite sharing the same margin value, headings are
qualitatively different from these other elements. Both in terms
of customisation and debugging using browser tools, it is cleaner
to keep the margins of heading separate.