Automates line-height calcs and simplifies use of border mixins

Really great work Ian, typesettings is very impressive.

I'm probably missing something vital, but it seems to me that the line-height calculation can be automated so you need only pass the font-size to the setType mixin and get the correct line-height.

This would mean a user cannot unintentionally bork the line-height by passing an inappropriate value to the mixin.