Implements garbage collection subcommand

- Includes a change in the command to run the registry. The registry
  server itself is now started up as a subcommand.
- Includes changes to the high level interfaces to support enumeration
  of various registry objects.

Signed-off-by: Andrew T Nguyen <andrew.nguyen@docker.com>

cmd/registry/main.go

@@ -20,5 +20,5 @@ import (
 )
 
 func main() {
-	registry.Cmd.Execute()
+	registry.RootCmd.Execute()
 }