Make Hero image work out-of-the box (#40)

* using Hero sample image in the theme's /static folder
* fall back to site config
* fall back to hard coded default if none set in page or site params

layouts/partials/site-header.html

-{{ $featured_image := .Params.featured_image }}
+{{ $featured_image := .Param "featured_image" | default "images/gohugo-default-sample-hero-image.jpg" }}
 {{ if $featured_image }}
   {{/* Trimming the slash and adding absURL make sure the image works no matter where our site lives */}}
   {{ $featured_image := (trim $featured_image "/") | absURL }}