feat: move NgZone to Stream/Observable-based callback API

BREAKING CHANGES:
- deprecates these methods in NgZone: overrideOnTurnStart, overrideOnTurnDone, overrideOnEventDone, overrideOnErrorHandler
- introduces new API in NgZone that may shadow other API used by existing applications.

modules/angular2/src/core/zone.ts

@@ -1,2 +1,2 @@
 // Public API for Zone
-export {NgZone} from './zone/ng_zone';
+export {NgZone, ZeroArgFunction, ErrorHandlingFn, NgZoneError} from './zone/ng_zone';