feat($http): adds xhrFields to $http config object

The properties of the `xhrFields` object is set on the XHR object making it possible to set non-standard properties. It is also possible to set the object on `$http.defaults` to use it globally. Please note that this will not work for certain non-standard read-only properties specified by Mozilla, such as `mosSystem` and `mozAnon` as they must be passed throught the constructor.