Prepend assets.base_url to liquid img tags.
Consolidate images into _assets/img/.
Actually, I can do this with three lines.
Report missing images and malformed tags.

Nonfatally.
HULK COMMIT

Replace /images/... with ... in all `{% img %}` tags.
More better regex improvements
Use `/` for images at the asset root.

Update existing img tags that do this.
Merge pull request #543 from rackerlabs/jekyll-images-url

Generate CDN URLs for liquid { img } tags