feat($http): add XHR events configuration option

Allows for arbitrary event handling on requests.

Author: chrisirhc

src/ng/http.js

@@ -1155,7 +1155,7 @@ function $HttpProvider() {
         }
 
         $httpBackend(config.method, url, reqData, done, reqHeaders, config.timeout,
-            config.withCredentials, config.responseType);
+            config.withCredentials, config.responseType, config.events);
       }
 
       return promise;

src/ng/httpBackend.js

@@ -28,7 +28,7 @@ function $HttpBackendProvider() {
 
 function createHttpBackend($browser, createXhr, $browserDefer, callbacks, rawDocument) {
   // 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, eventHandlers) {
     $browser.$$incOutstandingRequestCount();
     url = url || $browser.url();
 
@@ -88,6 +88,20 @@ function createHttpBackend($browser, createXhr, $browserDefer, callbacks, rawDoc
       xhr.onerror = requestError;
       xhr.onabort = requestError;
 
+      if (eventHandlers) {
+        forEach(eventHandlers, function (value, key) {
+          if (key !== 'upload') {
+            xhr.addEventListener(key, value);
+          }
+        });
+
+        if (eventHandlers.upload) {
+          forEach(eventHandlers.upload, function (value, key) {
+            xhr.addEventListener(key, value);
+          });
+        }
+      }
+
       if (withCredentials) {
         xhr.withCredentials = true;
       }