This is a follow-on to PR #62, and it borrows much of the format
from #993, but uses specific formats for the image manifest and manifest
list (fat manifest) instead of a combined generic format.
The intent of this proposed manifest format is to allow multi-arch, and
allow for full content-addressability of images in the Docker engine.
Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>
