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)