added stub plugins socket file for plugin usage (ping @schamper @frissdiegurke @akhoury @psychobunny @barisusakli)

src/socket.io/plugins.js

@@ -0,0 +1,16 @@
+'use strict';
+
+var	SocketPlugins = {};
+
+/*
+	This file is provided exclusively so that plugins can require it and add their own socket listeners.
+
+	How? From your plugin:
+
+		var SocketPlugins = module.parent.require('./socket.io/plugins');
+		SocketPlugins.myPlugin.myMethod = function() { ... };
+
+	Be a good lad and namespace your methods.
+*/
+
+module.exports = SocketPlugins;