Group pages within their own directory

Add `pages` as it's own content type, and move all page content into
the appropriate directory.

Any references to `layout` in the YAML front matter has also been
removed as it's implied by the content type. Except for `about.md`
which does explicity use a different layout.

app/config/sculpin_kernel.yml

@@ -2,6 +2,8 @@ sculpin:
     ignore: ['**/.DS*', '**/*.swp']
 
 sculpin_content_types:
+    pages:
+        permalink: /:basename/
     posts:
         permalink: articles/:basename/
         taxonomies: [tags]