Removed button backgrounds on static pages

The home and subscribe buttons had backgrounds on static pages, but not
post pages. The backgrounds are now removed from these buttons on
static pages.
Fixed bottom margin on static page post-title h1

Static pages were previously missing a margin on the bottom of the
post-title h1. By adding <header> with a post-header class around the
h1, we gain the exact same bottom margin as seen on post (non-static)
pages.
Changed button background fix to match post.hbs

As per JohnONolan’s suggestion, I have edited the nav to match the code
in post.hbs: https://github.com/TryGhost/Casper/blob/master/post.hbs#L9
Merge pull request #143 from cubb/master

Fixes to visual bugs on static pages
Merge branch 'master' of https://github.com/TryGhost/Casper