Rewrite ampersand-sync to use xhr

Thanks to @Raynos for implementing the beforeSend function, which made this possible!

I've done my best to make sure that it's pretty much completely backwards compatible, but it's possible there might be some differences still, so it might constitute a major version bump. If you have any feedback, let me know.

Goodbye jQuery!
![byebye](http://media.giphy.com/media/2FazhqpAddswFFImQ/giphy.gif)