Implement path spec for manifest storage
Move Manifest type into storage package

This changeset move the Manifest type 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.
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.
Merge pull request #780 from stevvooe/manifest-storage

Initial implementation of image manifest storage