static registry support

For a pull-only, static registry, there only a couple of headers that
need to be optional (that are presently required.
* X-Docker-Registry-Version
* X-Docker-Size
* X-Docker-Endpoints

To support current behavior, and re-use API, let's put version and standalone
information into the _ping. And to support Headers they are checked after the
JSON is loaded (if there is anything to load). To stay backwards compatible, if
the _ping contents are not able to unmarshal to RegistryInfo, do not stop, but
continue with the same behavior.

Conversation was had on list as well. https://groups.google.com/forum/#!topic/docker-dev/8SOGjSzK5Hg