$.ajax progress event

```js
function handleUploadProgress(event) {
  //... 
}

function handleDownloadProress(event) { 
  //...
}

function prepareXHR() {
  var xhr = new XMLHttpRequest();
  xhr.upload.addEventListener("progress", handleProgress, false);
  xhr.addEventListener("progress", handleProgress, false);
  return xhr;
};

$.ajax({
  type: 'POST',
  url: url,
  data: data,
  xhr: prepareXHR
})
  .done(handleDone)
  .fail(handleFail)
  .always(handleAlways);
```