Initial implementation of image manifest storage

This change implements the first pass at image manifest storage on top of the storagedriver. Very similar to LayerService, its much simpler due to less complexity of pushing and pulling images.

Various components are still missing, such as detailed error reporting on missing layers during verification, but the base functionality is present.

The Manifest type has been moved into the storage package to make the type accessible to client and registry without import cycles. The structure of the manifest was also changed to accuratle reflect the stages of the signing process. A straw man Manifest.Sign method has been added to start testing this concept out but will probably be accompanied by the more import SignedManifest.Verify method as the security model develops.

This is probably the start of a concerted effort to consolidate types across the client and server portions of the code base but we may want to see how such a handy type, like the Manifest and SignedManifest, would work in docker core.

Merging this PR into docker-registry:next-generation depends on #769