Move 'auth' to the registry subsystem

This is the first step towards separating the registry subsystem from
the deprecated `Server` object.

* New service `github.com/dotcloud/docker/registry/Service`
* The service is installed by default in `builtins`
* The service only exposes `auth` for now...
* ...Soon to be followed by `pull`, `push` and `search`.

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)