Ie8 support

IE8 really doesn't like it when you try to change/add the status or statusCode property of an xhr-instance. This PR fixes IE8 support by returning a new response object instead (so it's a breaking change).

I also changed it so that the callback now is optional.

Please let me know if this is something that you'd consider merging, or if I should create a separate module that has IE8 support.

Cheers!