Add option to trigger xhr.withCredentials without http auth

Firefox sends the username for auth even though a password was not supplied
which causes Firefox to abort before sending a preflight request for Cross-Origin
requests.  

This pull request adds a withCredentials request option that allows you to set ``xhr.withCredentials`` without sending http authentication.

See http://stackoverflow.com/questions/11255173/firefox-wont-send-cross-origin-resource-sharing-pre-flight
Why not just:

```js
if (this.options.user && this.options.password && 'withCredentials' in xhr) ...
```