Adding events and progress monitoring to Uize.Comm.Ajax

This Ajax module now fires three possible events, `XHR Progress`, `XHR Load`, and `XHR Error`. `XHR Progress` fires in the XMLHttpRequest's `onprogress` handler, if available. `XHR Load` and `XHR Error` fire when `onreadystatechange` fires at `readyState == 4`. `XHR Error` fires if the response code is not 200.

The events are fired with the respective data required for the most common use cases.

The changes should be fully backwards compatible.