Made the code more robust.

Extended functionality with helper functions.

bin/example/sample.php

@@ -0,0 +1,14 @@
+#!/usr/bin/php
+<?php
+
+use function Joop\Asynchronous\async;
+
+require_once __DIR__ . '/../../vendor/autoload.php';
+
+$process = function ($number) {
+	sleep($number);
+	return $number;
+};
+
+async($process, 2);
+// Do stuff...