how to use composer in opencart 2.2.0

Question

So, based on Opencart 2.2.0 installation guide :

From version 2.2 composer has been added to aid developers who want to use composer libraries. 2 versions of OpenCart will become available, one compiled and one non-compiled (composer.json only - no files in vendor folder). We STRONGLY advise leaving the vendor folder outside of the webroot - so files cannot be accessed directly.

I have this directory structure under /srv/www/mywebsite.com on my VPS.

.
├── composer.json
├── nbproject
├── public_html
│   ├── admin
│   ├── catalog
│   ├── image
│   ├── index.php
│   ├── libraries
│   │   ├── exec.php
│   │   ├── InterfaceREST.php
│   │   ├── kurir
│   │   ├── RestGuzzle5.php
│   │   └── Test.php
│   ├── system
│   │   ├── startup.php
│   └── vqmod
├── tests
└── vendor
    ├── guzzlehttp
    ├── psr

And this is my composer.json :

{
    "name": "opencart",
    "type": "project",
    "description": "online store",
    "keywords": ["opencart", "ecommerce", "framework", "opensource"],
    "homepage": "http://www.mywebsite.com",
    "license": "MIT",
    "authors": [
        {
            "name": "fatimah",
            "email": "fatimah@mywebsite.com"
        }
    ],
    "autoload": {
        "psr-4": {
            "GodFather\\": "libraries"
        }
    },
    "require": {
        "cardinity/cardinity-sdk-php": "^1.0",
        "braintree/braintree_php": "3.2.0",
        "leafo/scssphp": "0.0.12",
        "php": ">=5.4.0"
    }
}

I've create some classes under directory libraries.

Test.php (example)

<?php

namespace GodFather;

use GodFather\InterfaceREST;

class Test
{

    public function __construct() {

    }

    public function test_rest(InterfaceREST $rest) {
        $rest->request('get', '/kontak-kami');
        var_dump($rest->get_status_code());
    }

}

And I'm trying to use guzzle by running exec.php from cli. php -f exec.php

<?php

require_once('../index.php');

use GuzzleHttp\Client;

$rest = new Client(['base_url' => 'http://zuwaily.blogspot.co.id']);
$req = $rest->createRequest('get', '/p/hubungi-zuwaily.html');
$res = $rest->send($req);
echo $res->getStatusCode();

I don't get status code.

How to exactly use composer in Opencart? And how to use psr-4? am I doing correct? I need your advise.

Thanks.