Using composer in extensions

composer.json

{
    "name": "3sd/whizzy",
    "description": "Blah blah blah...",
    "authors": [
        {
            "name": "Michael McAndrew",
            "email": "michaelmcandrew@thirdsectordesign.org"
        }
    ],
    "require": {
        "simshaun/recurr": "^2.2"
    }
}

CRM_Whizzy_Page_Whiz.php

<?php
class CRM_Whizzy_Page_Whiz extends CRM_Core_Page {

  public function run() {
    $rule = new \Recurr\Rule;
    parent::run();
    
    // etc...

whizzy.php

<?php
require __DIR__ . '/vendor/autoload.php';
require_once 'whizzy.civix.php';

/**
 * Implements hook_civicrm_config().
 *
 * @link http://wiki.civicrm.org/confluence/display/CRMDOC/hook_civicrm_config
 */
function whizzy_civicrm_config(&$config) {
  _whizzy_civix_civicrm_config($config);
}

// etc...