feat($http): add notify callback for loading requests

src/ng/http.js

@@ -991,7 +991,7 @@ function $HttpProvider() {
         }
 
         $httpBackend(config.method, url, reqData, done, reqHeaders, config.timeout,
-            config.withCredentials, config.responseType);
+            config.withCredentials, config.responseType, notify);
       }
 
       return promise;
@@ -1026,6 +1026,14 @@ function $HttpProvider() {
       }
 
 
+      /**
+       * Notify callback registered to $httpBackend()
+       */
+      function notify(xhr) {
+        deferred.notify(xhr);
+      }
+
+
       /**
        * Resolves the raw $http promise.
        */

src/ng/httpBackend.js

@@ -40,7 +40,7 @@ function createHttpBackend($browser, createXhr, $browserDefer, callbacks, rawDoc
   var ABORTED = -1;
 
   // TODO(vojta): fix the signature
-  return function(method, url, post, callback, headers, timeout, withCredentials, responseType) {
+  return function(method, url, post, callback, headers, timeout, withCredentials, responseType, notify) {
     var status;
     $browser.$$incOutstandingRequestCount();
     url = url || $browser.url();
@@ -103,6 +103,8 @@ function createHttpBackend($browser, createXhr, $browserDefer, callbacks, rawDoc
               response,
               responseHeaders,
               statusText);
+        } else if (xhr && xhr.readyState == 3 && notify) {
+          notify(xhr);
         }
       };