Various Minor Changes

Updated `(FsLayer).hash` method.
Exposed `(Image).NormalForm` method.
Added methods for Encoding/Decoding Image manifests.
Reorganized `PortSpec` type to implement `sort.Interface`.
Updated `(RuntimeParams).hash` method with `PortSpec` change.

Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)