Add types.ImageSource and types.ImageDestination
Add types.ImageSource and types.ImageDestination

Right now, only a declaration.

This will allow writing generalized push/pull between various storage
mechanisms, and reuse of the Docker Registry client code for the Docker
Registry embedded in OpenShift.

containers/image,mikebrow/skopeo,masters-of-cats/image,mtrmac/skopeo,mtrmac/skopeo,mikebrow/skopeo,containers/image,jwhonce/skopeo,mtrmac/skopeo,projectatomic/skopeo,jwhonce/skopeo,projectatomic/skopeo