fix($q): Use extend to avoid overwriting prototype

This patch uses `extend()` to add prototypal methods to `Promise` and `Deferred`, to avoid having to set `constructor` manually on the overwritten prototype.

Author: bluepnume

src/ng/q.js

@@ -270,9 +270,7 @@ function qFactory(nextTick, exceptionHandler) {
     this.$$state = { status: 0 };
   }
 
-  Promise.prototype = {
-    constructor: Promise,
-    
+  extend(Promise.prototype, {
     then: function(onFulfilled, onRejected, progressBack) {
       var result = new Deferred();
 
@@ -294,7 +292,7 @@ function qFactory(nextTick, exceptionHandler) {
         return handleCallback(error, false, callback);
       }, progressBack);
     }
-  };
+  });
 
   //Faster, more basic than angular.bind http://jsperf.com/angular-bind-vs-custom-vs-native
   function simpleBind(context, fn) {
@@ -341,9 +339,7 @@ function qFactory(nextTick, exceptionHandler) {
     this.notify = simpleBind(this, this.notify);
   }
 
-  Deferred.prototype = {
-    constructor: Deferred,
-    
+  extend(Deferred.prototype, {
     resolve: function(val) {
       if (this.promise.$$state.status) return;
       if (val === this.promise) {
@@ -407,7 +403,7 @@ function qFactory(nextTick, exceptionHandler) {
         });
       }
     }
-  };
+  });
 
   /**
    * @ngdoc method