new responsive nav

major restructure of html and css to accomodate footer anchor pattern
navigation
fix for excerpt-index (overflow: auto) and headings margin fix

- Overflow: auto CSS absolutely necessary for determining a fixed
height for the excerpt-index.
- Headings in Chrome were ignoring the headings margin reset! I'm not
sure why this was a bug. Seemed to be introduced in Chrome 28.
- Revert to excerpt-index not being inside a section
new responsive menu for small screens, improved padding for large screens
.banner is now #banner, "back to top" fixed
make sure that `body` does not determine the background

This will greatly simplify maintainability and wlll allow for reusable
or replaceable backgrounds. A lone styled div with class "background"
now sets the background for the whole site.
updated implementation details on the readme

More details about content-first
Merge branch 'body-fix' into new-main-navigation
padding improvements for menus in banner
body margin fix, increase all .1 opacity on elements to .2

Thanks @jeancflanagan for the opacity contrast accessibility suggestion!
banner h1 scaling, nav/menu padding improvements
adjustment for better scaled prev/next page navigation
positioning fix for large screen menu
Merge pull request #28 from opattison/new-main-navigation

New main navigation (resolves #23, #27)