Handle XHR response error

[BehindTheMath]

• Move the XHR callback to a separate file
• Trigger an error event if the response cannot be parsed.

Fixes #123.

[robinnorth]

This needs to be tempOptions = clone(this.options), or you'll mutate the current instance's options

[BehindTheMath]

Good point.

[robinnorth]

This also needs to clone this.options, but you could probably just do this once at the top of the method rather than repeating the assignment of the request to your temporary event options.

[BehindTheMath]

Good point.

[robinnorth]

It's all looking good to me now, do you want to add some tests for handle-response?

[BehindTheMath]

Yes, please. You're better at that than I am.

[robinnorth]

I don't have much time to do that at present, unfortunately, so you can either have a go yourself or, as handle-response is largely a direct transplant from index.js and as most of the remainder of that file is untested as well, we could park adding tests for this until #21 if you want to get this merged soon.

[robinnorth]

Thanks for adding tests, I'll try and find some time to review them shortly.

[robinnorth]

Took a look at the tests this morning, all seems good to merge to me!