Revert "Guard against Middleman::Sitemap::Resource during build generation"

This reverts commit 9183e1e55dc498c07357c033a2d224630d3924c1.
Cleanup and consolidation of _main.css.scss
Consolidate #header styles in _main.css.scss
Remove unused grid CSS files
Remove unused h5bp CSS files
Extract out settings CSS into separate include
Rename screen.css to application.css for better consistency with rails
Remove unused responsive.css / media folder
Move stylesheets/plugins to stylesheets/vendor for better consistency with rails
Remove _responsive.css.scss
Remove unused _parts.css.scss
Various cleanup/organization of stylesheets/parts
Remove unused syntax.scss / twitter.scss
Minor tweaks to styles of sidebar