Support for Serving on Generic net.Listener

[adrianosela]

Support for Serving on Generic net.Listener

Adds the ability to initialize a VncProxy object with your own net.Listener.

Example:

Where ln is a net.Listener implementation

	proxy := &vncproxy.VncProxy{
		NetListener: ln,
		ProxyVncPassword: "server-password",
		SingleSession: &vncproxy.VncSession{
			TargetHostname: "127.0.0.1",
			TargetPort:     "5900",
			TargetPassword: "target-password"
			ID:             "dummySession",
			Status:         vncproxy.SessionStatusInit,
			Type:           vncproxy.SessionTypeProxyPass,
		},
		UsingSessions: false,
	}

This is useful to me because I have a listener that does end to end encryption and other additional functionality. It's basically a TCP connection with LOTS of middlewares - abstracted as a net.Listener. I would love for the vnc proxy to accept connections over this net.Listener.

[adrianosela]

@amitbet it would mean a lot to me for this change to be reviewed :) thanks for the awesome work.