Move gh-pages branch files into /docs and add test files

- Jekyll ignore `/docs` and `/test` folders when using from root
- Update Staticman config to point to correct branch and data file location
- Replace `{{ base_path }}` references with `absolute_url` filter
- Update documentation

test/_posts/2012-03-15-layout-header-image-external.md

@@ -0,0 +1,17 @@
+---
+title: "Layout: Header Image (External URL)"
+header:
+  image: https://farm5.staticflickr.com/4140/4939863887_84705982fd_b.jpg
+categories:
+  - Layout
+  - Uncategorized
+tags:
+  - edge case
+  - featured image
+  - image
+  - layout
+---
+
+This post should display a **header image**, if the theme supports it.
+
+Featured image is an external asset and should load.