Compatibility with express 3.x

nodejs-munin fails if used with express 3.x:
```
/var/www/ha-git/tools/munin/node_modules/munin/lib/munin-server.js:38
    console.log("Initialized munin stats server:", server.address().address, '
                                                          ^
TypeError: Object function app(req, res){ app.handle(req, res); } has no method 'address'
    at Server.module.exports (/var/www/ha-git/tools/munin/node_modules/munin/lib/munin-server.js:38:59)
    at Server.g (events.js:192:14)
    at Server.EventEmitter.emit (events.js:93:17)
    at Server._listen2 (net.js:923:10)
    at process.startup.processNextTick.process._tickCallback (node.js:244:9)
```
I think it's a related bug: https://github.com/visionmedia/express/issues/1136