add object pool pattern

Pool/Worker.php

@@ -0,0 +1,22 @@
+<?php
+
+namespace DesignPatterns\Pool;
+
+class Worker
+{
+
+    public function __construct()
+    {
+        // let say that constuctor does really expensive work...
+        // for example creates "thread"
+    }
+
+	public function run($image, array $callback)
+	{
+        // do something with $image...
+        // and when it's done, execute callback
+        call_user_func($callback, $this);
+	}
+
+}
+