fix(core): always remove DOM listeners and stream subscriptions

This is needed to prevent memory leaks. The DOM
listeners don’t need to be removed for simple examples,
but a big internal app shows memory leaks because of them.

BREAKING CHANGE:
- `Renderer.listen` now has to return a function that
  removes the event listener.

modules/angular2/src/core/render/api.ts

@@ -28,7 +28,7 @@ export abstract class Renderer implements ParentRenderer {
 
   abstract destroyView(hostElement: any, viewAllNodes: any[]);
 
-  abstract listen(renderElement: any, name: string, callback: Function);
+  abstract listen(renderElement: any, name: string, callback: Function): Function;
 
   abstract listenGlobal(target: string, name: string, callback: Function): Function;