Implemented per-page or site-wide background images.

If the user sets "background: /path/to/image.jpg" or an http(s) url in their _config.yml, or if they set it in a post/page's yaml front matter, the displayed page will use that image as the background. If their background is transparent, the default background color will also be visible. Setting a background in a page/post will take precedence over the one set in _config.yml.
Implemented mmistakes' suggestions.

All background images will now be consistent with the rest of the theme
and pull background images from the site's /images location.

Also moved per-page background yaml declaration into the images: "group"
or array thing (whatever it's called). This is also consistent with the
rest of the theme.
Add sample post demoing background image
Add background image to site
Document background image feature
Merge branch 'release/1.2.0' into develop