Remove duplicate override of sup header in page_for_posts area

Introduced back in April by me
Adjust spacing
Trim global headers before testing for blankness

We won't go as far as checking empty, as it's possible (???) somebody
wants to use 0 one day. :)
Remove the page for posts title as a sup header
Categories no longer behave differently than other archives
No longer setting a different sup_header at archive level
Document behavior
Use site_tagline as default for sub on front page
Merge pull request #83 from washingtonstateuniversity/main-header

[PR] Adjustments to Main Header sup and sub