Added option to ajax to include additional  xhrFields, e.g. add withCredentials = true;

Example usage:
```
var        options = {
                                url: 'someurl.php',
                                method: 'POST',
                                cache: true,
                                data: someObject,
                                xhrFields: {
                                    withCredentials: true
                                },
                                crossDomain: true
                            };

                        return $$.ajax(options);
```