refactor(router): do not export RootRouter

BREAKING CHANGE:

It's unlikely that any apps were explicitly referencing `RootRouter`, but if they were they should
prefer to use the `routerBindings` helper or the `ROUTER_BINDINGS` const exported from `angular2/router`

modules/angular2/router.ts

@@ -4,7 +4,7 @@
  * Maps application URLs into application states, to support deep-linking and navigation.
  */
 
-export {Router, RootRouter} from './src/router/router';
+export {Router} from './src/router/router';
 export {RouterOutlet} from './src/router/router_outlet';
 export {RouterLink} from './src/router/router_link';
 export {RouteParams} from './src/router/instruction';