Adding a benchmark for websockets

Motivation:

It is often helpful to measure the performance of connections, e.g. the
latency and the throughput. This can be performed through benchmarks.

Modification:

This adds a simple but configurable benchmark for websockets into the
example directory. The Netty WebSocket server will echo all received
websocket frames and will provide an HTML/JS page which serves as the
client for the benchmark.

Result:

A benchmark is added in form a further Netty websocket example.
With this benchmark it is easily possible to measure the performance between Netty and a browser

This is how it looks like:

![Screenshot](http://abload.de/img/benchkleg1.png)
You mean changing it into `ctx.write(frame.retain());`?