New error type
WIP Image integration with Kubernetes

This is modelled as a simple plugin - adding the ability to record image metadata for use by other components within Kubernetes (pod generators, etc).  A repository object can keep tags, references to images, and a default set of override metadata with the image.

The demonstrated integration is with a Docker registry that has been configured to a) tag images with their IDs on push and b) notify Kubernetes via API that a new image has been tagged.  This allows clients to `docker push <registry>/foo/bar` and have `foo/bar` be updated in Kubernetes transparently.  See https://github.com/smarterclayton/docker-registry/compare/kubernetes_integration

This does not yet, but will, support a pass through / cache mode for a real Docker registry allowing consumers to map multiple real registries into a single namespace and to lazily instantiate data.