add stream handler

i added the stream handler. it is easy to handle streaming event (`monitor` or `stream`) interface. following code is example:

```javascript
var client = require('node-serf').connect({port: 7373}, function() {
  var handler = client.monitor({LogLevel: 'DEBUG'}); // returns instance of stream handler

  handler.on('data', function(result) {
    // handle streaming message
    console.log(result);
  });

  setTimeout(function() {
    // stop streaming
    handler.stop();
  }, 1000);
});
```

thanks.