registry: Added simple checksums (sha256) for layers

Docker-DCO-1.1-Signed-off-by: Sam Alba <sam@docker.com> (github: samalba)
registry: Fixed tests

Docker-DCO-1.1-Signed-off-by: Sam Alba <sam@docker.com> (github: samalba)
registry: Removed checksumPayload from exported fields

Docker-DCO-1.1-Signed-off-by: Sam Alba <sam@docker.com> (github: samalba)
registry: Fixed unexported field

Docker-DCO-1.1-Signed-off-by: Sam Alba <sam.alba@gmail.com> (github: samalba)
Merge pull request #4297 from dotcloud/add-simple-checksum

registry: Added simple checksums (sha256) for layers