Make server port a command-line option in the socket.io example.

In playing with the socket.io example, the strength of RabbitMQ is not really clear when you are only running one server.  This makes the port a command-line option so you can easily start up 4-5 servers on different ports and watch as your messages are passed from one browser tab to another, on completely different servers.

Basically you get to see a message move along the following path:

Browser -> Socket.IO -> Node.js -> RabbitMQ -> Node.js -> Socket.IO -> Browser

Pretty impressive for a first-timer :)

I'm not sure of the best place to document this intended test scenario.