Make it possible to pass arguments to the xhr object

With Firefox it is possible to pass additional arguments to `XMLHttpRequest` (e.g. to enable [`systemXHR`](https://developer.mozilla.org/en-US/Apps/Reference#systemXHR) under Firefox OS).

These parameters are explained [here](https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest#Non-standard_properties).

This change makes it possible to specify arguments to `XMLHttpRequest` by setting `params.xhrParams`.