Enhancement to asyncValidator callbacks

Went with `fn.call(this, xhr)` instead of `fn.apply(this, [xhr])` - there's only 1 argument.
You can now access the `ParsleyField` plugin via `this` inside the validators, should you need to run any post-AJAX stuff before returning validity.

Thought i squashed the commits better, but they were probably from a previous push before i learned how to do that.

Let me know if you have questions or concerns. Thanks for the help!