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

The `xhrFields` object is passed to the XMLHttpRequest constructor making it possible to set non-standard properties on the XMLHttpRequest object. It is also possible to set the object on `$http.defaults` to use it globally. With this functionality it is possible to set the `mozSystem` and `mozAnon` properties, which is required to use angular in a packaged Firefox OS app. See https://developer.mozilla.org/en-US/docs/DOM/XMLHttpRequest#XMLHttpRequest()

Closes #2318