If request is cancelled or returns 0 fire callback

I added this code in so that if readystate === 4 and status === 0 if fires the callback.
This enables it to work offline and in kiosk mode.
Update ajax.js

updated so that it fires the callback if the status is < 400
Merge pull request #15 from sidonaldson/master

If request is cancelled or returns 0 fire callback