Retwis ported to modern Redis data types.

The example (and related documentation at redis.io) was not updated for
5 years. Redis had no sorted sets and hashes when the original code was
written (!). An update was really needed.

We also use a modern PHP client now: Predis. A copy is shipped within
this repository to make life easier to newcomers trying Redis for the
first time via this example.

Predis/ExecutableContextInterface.php

@@ -0,0 +1,29 @@
+<?php
+
+/*
+ * This file is part of the Predis package.
+ *
+ * (c) Daniele Alessandri <suppakilla@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Predis;
+
+/**
+ * Defines the interface of a basic client object or abstraction that
+ * can send commands to Redis.
+ *
+ * @author Daniele Alessandri <suppakilla@gmail.com>
+ */
+interface ExecutableContextInterface
+{
+    /**
+     * Starts the execution of the context.
+     *
+     * @param  mixed $callable Optional callback for execution.
+     * @return array
+     */
+    public function execute($callable = null);
+}