Add server Data to progress event

There is no point in having a progress event if I can't see the progress value.

```js
var progressListener = new function() {
  this.onProgress = function(task, status, serverData) {
    if(serverData && serverData.progress) {
      console.log(status + ' ' + serverData.progress + '%');
    }
  }
}

task.addListener(progressListener);
```