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.
Merge pull request #5 from Robpol86/master

Implemented per-page or site-wide background images.