refactor(async): refactor EventEmitter

Refactor EventEmitter and Async Facade to match ES7 Observable semantics, properly use RxJS typedefs, make EventEmitter inherit from RxJS Subject. Closes #4149.

BREAKING CHANGE:
- consumers of EventEmitter no longer need to call .toRx()
- EventEmitter is now generic and requires a type - e.g. `EventEmitter<string>`
- EventEmitter and Observable now use the `.subscribe(generatorOrNext, error, complete)` method instead of `.observer(generator)`
- ObservableWrapper uses `callNext/callError/callComplete` instead of `callNext/callThrow/callReturn`

modules/playground/src/zippy_component/zippy.ts

@@ -7,8 +7,8 @@ import {ObservableWrapper} from 'angular2/src/core/facade/async';
 export class Zippy {
   visible: boolean = true;
   title: string = '';
-  openHandler: EventEmitter = new EventEmitter();
-  closeHandler: EventEmitter = new EventEmitter();
+  openHandler: EventEmitter<any> = new EventEmitter();
+  closeHandler: EventEmitter<any> = new EventEmitter();
 
   toggle() {
     this.visible = !this.visible;