Update for compatibility with Express 3.1

Express 3.x does not return an http server instance, and this is the
updated method for doing the same thing.

app.js

@@ -1,7 +1,10 @@
-var app = require('express').createServer()
-var io = require('socket.io').listen(app);
+var express = require('express')
+  , app = express()
+  , http = require('http')
+  , server = http.createServer(app)
+  , io = require('socket.io').listen(server);
 
-app.listen(8080);
+server.listen(8080);
 
 // routing
 app.get('/', function (req, res) {