Setting up a vertical rhythm for typography with compass

This uses the compass vertical rhythm mixin to setup the typography settings
in addition to modular scale. The fonts and line-heights are set in pixels to
make this work sensibly. When Compass changes to support rems with a pixel
fall back that can be used instead.

I've also added class based font sizes so that font sizes can be used 
regardless of header level. It isn't all that common that H1 is always the
same size in all contexts and with the new HTML5 sematics it will be pretty
common for most headings in a hierarchy of content to be H1's.