Add a dummy plugin

Makes it easier to update plugins when the API has been severly changed
between releases (just point "cryptal.entrypoint" to this dummy plugin).

tests/DummyPlugin.php

@@ -0,0 +1,14 @@
+<?php
+
+namespace fpoirotte\Cryptal\Tests;
+
+use fpoirotte\Cryptal\Implementers\PluginInterface;
+use fpoirotte\Cryptal\RegistryWrapper;
+
+class DummyPlugin implements PluginInterface
+{
+    public static function registerAlgorithms(RegistryWrapper $registry)
+    {
+        // Do nothing
+    }
+}