Header fix

Make header small & fixed in all pages expect homepage (fixes #166)
While fixing header height did some refactoring:
 - DRY some jade files that are now in common `partials` instead of duplicated
 - removed mixin's in Stylus (used to force different colors in each project)
 - re-organize variables `site`, `navigation` & `page` sent to Jade.
 - CSS cleanup. Removed old stuff not used anymore, made all color codes small case (fixes #189)