message: refactor(groupBy): put back export of GroupedObservable so-as not to break existing API
diff: import { Observable } from '../Observable';
  import { Subject } from '../Subject';
  import { groupBy as higherOrder, GroupedObservable } from '../operators/groupBy';
  +export { GroupedObservable };
  /* tslint:disable:max-line-length */
  export function groupBy<T, K>(this: Observable<T>, keySelector: (value: T) => K): Observable<GroupedObservable<K, T>>;
  