feat(ngFor): add custom trackBy function support

Make it possible to track items in iterables in custom ways (e.g. by ID or index), rather than simply by identity.

Closes #6779

modules/angular2/src/core/change_detection.ts

@@ -20,5 +20,6 @@ export {
   IterableDifferFactory,
   KeyValueDiffers,
   KeyValueDiffer,
-  KeyValueDifferFactory
+  KeyValueDifferFactory,
+  TrackByFn
 } from './change_detection/change_detection';