client: add xhr2 file download

adds ability for the client to download files as ```arraybuffer``` or ```blob```:

```javascript
request.parse['myFileMimetype'] = function(obj) {
  return obj;
}

request
.get('/arraybuffer')
.on('request', function () {
  this.xhr.responseType = 'arraybuffer'; // or blob
})
.end(function(res) {
  // now res.body is an arraybuffer or a blob
});
```