feat($http): add option for unsafe xhr requests in FirefoxOS

Previously, it was not possible to pass settings to the constructor of the XMLHttpRequest.  This
adds an optional config property `xhrConfig` to the $http constructor, allowing us to do so.  This
is required for use of the mozAnon and mozSystem options for FirefoxOS.
See https://developer.mozilla.org/en-US/docs/DOM/XMLHttpRequest#XMLHttpRequest() for details.

Closes #2318