Allow hyphens in namespaces.

We have some cases using a private registry where it would be convenient to be able to use hostnames and other strings that can include "-" as namespaces. The [original discussion about the acceptable characters in namespaces](https://github.com/docker/docker/pull/663#issuecomment-18437921) seemed a bit ambivalent on whether hyphens should be accepted in namespaces, so we'd like to propose voting them in.

This won't establish precedent for namespaces that aren't valid Docker hub usernames. The Docker client [accepts underscores in namespaces](https://github.com/docker/docker/blob/c478143ec0ca8f610f7980991b9b2ba0e1160b3e/registry/registry.go#L27), even though the Docker hub (despite [documentation to the contrary](https://docs.docker.com/reference/api/docker-io_api/#user-register)) doesn't allow them in usernames. Trying to create `test_user` today returns `Wrong username format (it has to match "^[a-z0-9]{4,30}$")`.

Signed-off-by: Matthew Riley <mattdr@google.com>
Perhaps more importantly, these patterns are possible with the client today. Even if preventing them were in the scope of this change, we would risk breaking private registry users that have namespaces including these patterns.