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)
Move 'search' to the registry subsystem

This continues the effort to separate all registry logic from the
deprecated `Server` object.

* 'search' is exposed by `github.com/dotcloud/docker/registry/Service`
* Added proper documentation of Search while I was at it

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)
remove fixme

Docker-DCO-1.1-Signed-off-by: Victor Vieux <vieux@docker.com> (github: vieux)
Merge pull request #5575 from vieux/pr-5428