Simplify OpenSeadragon.makeAjaxRequest

* Since async is always true – and browsers are starting to deprecate
  synchronous XHR – we were able to prune considerable amount of code
* Add an error callback to match the existing success callback
Better match project style
makeAjaxRequest: log all errors, test callback

* All AJAX errors will log to the console
* The onError callback will only be called if defined
makeAjaxRequest: throw error for missing callback
makeAjaxRequest: don't set request to null

Setting onreadystatechange can avoid the possibility of it somehow being
called twice but there's no point in nulling the request right before we
let the GC collect it.
Merge pull request #149 from acdha/simplified-ajax-requests

Simplify OpenSeadragon.makeAjaxRequest