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>
If your the type of person that names your image this then you have to accept all the bad things that will happen, i would prevent it, it's not worth it and if you have this then what did you expect to happen.