feat(change_detection): allow triggering CD form ChangeDetectorRef

Closes #4144

modules/angular2/src/core/change_detection/change_detector_ref.ts

@@ -25,6 +25,8 @@ export class ChangeDetectorRef {
    */
   detach(): void { this._cd.mode = ChangeDetectionStrategy.Detached; }
 
+  detectChanges(): void { this._cd.detectChanges(); }
+
   /**
    * Reattach the change detector to the change detector tree.
    *