if supplied, server.config will be passed to server

Good point about passing the callback. But, the current version of the ```log``` function (from lines 61-63) is:

```coffeescript
  log = ->
    logger.info "application started on http://localhost:#{port}/"
    callback()
```

The problem is that the custom server might not launch on ```localhost:#{port}``` (the custom server might listen on 0.0.0.0 or 127.0.0.2, or use a different value for ```port```).

I send another change to address this.