Listen callback does not fire until first connection

When using vantage in standalone mode, the listen callback does not fire until I connect to it with the command line client.

```
var Vantage = require('vantage');
var vantageServer = new Vantage();

vantageServer
.delimiter('$')
.listen(2001, function() {
console.log('Started vantage server at http://localhost:2001');
});
```