Implement a plugin manager and interface, update installer plugin tests

src/Composer/Plugin/PluginManager.php

@@ -0,0 +1,49 @@
+<?php
+
+/*
+ * This file is part of Composer.
+ *
+ * (c) Nils Adermann <naderman@naderman.de>
+ *     Jordi Boggiano <j.boggiano@seld.be>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Composer\Plugin;
+
+use Composer\Composer;
+use Composer\Package\PackageInterface;
+
+/**
+ * Plugin manager
+ *
+ * @author Nils Adermann <naderman@naderman.de>
+ */
+class PluginManager
+{
+    protected $composer;
+
+    protected $plugins = array();
+
+    /**
+     * Initializes plugin manager
+     *
+     * @param Composer $composer
+     */
+    public function __construct(Composer $composer)
+    {
+        $this->composer = $composer;
+    }
+
+    /**
+     * Adds plugin
+     *
+     * @param PluginInterface $plugin plugin instance
+     */
+    public function addPlugin(PluginInterface $plugin)
+    {
+        $this->plugins[] =  $plugin;
+        $plugin->activate($this->composer);
+    }
+}