docker save: ability to save multiple images

Now from a single invocation of `docker save`, you can specify multiple
images to include in the output tar, or even just multiple tags of a
particular image/repo.

```
> docker save -o bundle.tar busybox ubuntu:lucid ubuntu:saucy fedora:latest
> tar tf ./bundle.tar | wc -l
42
> tar xOf ./bundle.tar repositories
{"busybox":{"latest":"2d8e5b282c81244037eb15b2068e1c46319c1a42b80493acb128da24b2090739"},"fedora":{"latest":"58394af373423902a1b97f209a31e3777932d9321ef10e64feaaa7b4df609cf9"},"ubuntu":{"lucid":"9cc9ea5ea540116b89e41898dd30858107c1175260fb7ff50322b34704092232","saucy":"9f676bd305a43a931a8d98b13e5840ffbebcd908370765373315926024c7c35e"}}
```

Further, this fixes the bug where the `repositories` file is not created
when saving a specific tag of an image (e.g. ubuntu:latest)

document multi-image save and updated API docs

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)

api/server/server.go

@@ -611,10 +611,18 @@ func getImagesGet(eng *engine.Engine, version version.Version, w http.ResponseWr
 	if vars == nil {
 		return fmt.Errorf("Missing parameter")
 	}
+	if err := parseForm(r); err != nil {
+		return err
+	}
 	if version.GreaterThan("1.0") {
 		w.Header().Set("Content-Type", "application/x-tar")
 	}
-	job := eng.Job("image_export", vars["name"])
+	var job *engine.Job
+	if name, ok := vars["name"]; ok {
+		job = eng.Job("image_export", name)
+	} else {
+		job = eng.Job("image_export", r.Form["names"]...)
+	}
 	job.Stdout.Add(w)
 	return job.Run()
 }
@@ -1105,6 +1113,7 @@ func createRouter(eng *engine.Engine, logging, enableCors bool, dockerVersion st
 			"/images/json":                    getImagesJSON,
 			"/images/viz":                     getImagesViz,
 			"/images/search":                  getImagesSearch,
+			"/images/get":                     getImagesGet,
 			"/images/{name:.*}/get":           getImagesGet,
 			"/images/{name:.*}/history":       getImagesHistory,
 			"/images/{name:.*}/json":          getImagesByName,