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>

docs/root.go

@@ -0,0 +1,28 @@
+package registry
+
+import (
+	"github.com/docker/distribution/version"
+	"github.com/spf13/cobra"
+)
+
+var showVersion bool
+
+func init() {
+	RootCmd.AddCommand(ServeCmd)
+	RootCmd.AddCommand(GCCmd)
+	RootCmd.Flags().BoolVarP(&showVersion, "version", "v", false, "show the version and exit")
+}
+
+// RootCmd is the main command for the 'registry' binary.
+var RootCmd = &cobra.Command{
+	Use:   "registry",
+	Short: "`registry`",
+	Long:  "`registry`",
+	Run: func(cmd *cobra.Command, args []string) {
+		if showVersion {
+			version.PrintVersion()
+			return
+		}
+		cmd.Usage()
+	},
+}