Normalize headings

There are various inconsistencies surrounding headings that make
this change worth trialing. The font-size of headings in IE6/7
isn't relative to the root font-size (see #61). Modern browsers
set the `h1` font-size based on the depth of nesting in certain
HTML5 sectioning elements. This change overcomes both the issues.

At the same time, the margins are being normalized so that they
are consistent and `em`-based. If people need to customise the
heading font-size and margin, they can either edit normalize.css
directly or override later in the source. But at least any non-
customised headings will behave consistently now.

Close #41