feat(facade): do not reexport Observable from angular2/core

BREAKING CHANGE

Before import {Observable} from 'angular2/core'

After import {Observable} from 'rxjs/Observable';

modules/angular2/test/public_api_spec.ts

@@ -1010,11 +1010,6 @@ var NG_CORE = [
    RxJS API - may need to maintain as RxJS evolves
   */
   'EventEmitter.emit():js',
-  'Observable:js',
-  'Observable#create():js',
-  'Observable.forEach():js',
-  'Observable.lift():js',
-  'Observable.subscribe():js',
   'OutputMetadata',
   'OutputMetadata.bindingPropertyName',
   'enableDevMode():js',