implement pipeline expansion mechanism

I have implemented a pipeline expansion mechanism which is flexible and doesn't modify the framework logic flow. I think that pipeline customization freedom is needed in this framework because such freedom only adds great power to it and its usage isn't forced on anyone.

An use case would be adding a static file handler for compact interface serving or any other useful handler avoiding the need to instantiate another server or web container.

The pipeline customization is really easy with this method:

```
SocketIOChannelInitializer pipelineFactory = server.getPipelineFactory();
PipelineExtensionWrapper pexw = new PipelineExtensionWrapper("HttpStaticHandler", Mode.ADD_BEFORE, SocketIOChannelInitializer.PACKET_HANDLER, new HttpServerHandler(true));
pipelineFactory.getExtensionList().add(pexw);
```