[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 for new images.

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

The key use case is providing image metadata for use with other, higher level components, and ensuring that immutable references are available for historical image data.  The docker client cannot pull anything other than tags today (see Docker issue 7262 for a discussion), therefore any image that a client wants to repeatably deploy must be uniquely tagged.  In this case, the registry is ensuring that pushed images are tagged with their ID although clients could coordinate that on their own.

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.
I see. I wonder if maybe it would be worth cloning the net/url package and changing just that logic. I always feel like parsing stuff usually has unexpected edge cases and that by the time we finish getting it perfect, we'll have basically done that anyway.