Add WebSockets support

I have implemented basic websockets support. With example Echo adapter. It bases on my update-jetty branch.

Config option:
```mizuno websocket_app.ru --websockets "Mizuno::Websockets::Adapter::Echo"```

```Mizuno::Websockets::Adapter::Echo``` should be required somewhere, i.e. in config.ru

I think it's better to implement alternative .rb based config like in puma. Because in websockets case we might want to bind different adapters on different endpoints and it would be cumbersome with command line arguments