feat(Operator): Expose the Operator interface to library consumers

By exposing the Operator interface directly, consumers of Rx.ts can more easily
implement their own operators by calling lift() with their own operator.

Author: Kevin van der Vlist

src/Rx.ts

@@ -106,6 +106,7 @@ import './add/operator/zip';
 import './add/operator/zipAll';
 
 /* tslint:disable:no-unused-variable */
+import {Operator} from './Operator';
 import {Subscription} from './Subscription';
 import {Subscriber} from './Subscriber';
 import {AsyncSubject} from './subject/AsyncSubject';
@@ -138,6 +139,7 @@ export {
     Subject,
     Scheduler,
     Observable,
+    Operator,
     Subscriber,
     Subscription,
     Symbol,