Make createServer API be consistent with httpServer on node core

I was trying to use `dnsd` like I normally call my http node server:

``` javascript
var dnsd = require('dnsd'),
      port = 3737

dnsd.createServer(function (req, res) {
  res.end('1.2.3.4')
}).listen(port, function () {
  console.log('started on %s', port) 
})
```

`dnsd` was wrongfully using the callback function as the ip, which caused side effects. 

This patch prevents this behavior and re-introduces the same api as node http create server.

I couldn't find how to add tests and documentation for this change, but will provide them at your request