Site content changes

Added new includes and layouts. There is now a nav include, a
home-page-specific layout, and a collection page.
Add futura to bourbon

$futura is now a globally-available variable in scss
Grid layout adjustments

Added an $xlarge-screen breakpoint
Full bleed background

Added this as a stand-alone css module
Added logo file to nav
CSS updates

Home page layout is now generally working
Config YML tweaks

Expanded the list of items for Jekyll to exclude when building. Also
disabled pretty permalinks to keep relative URLs easier to keep track
of for now