RxJS: updated to version 2.2.15

* Core
 * renamed `Internals` to `internals`;
 * moved promises from Async to Core;
 * added `config` with promises configuration;
 * added `Observable.fromGenerator` method;
 * added overloads for `merge`, `concat`, `selectMany`/`flatMap` taking promises instead of observables;
* Async
 * added tests for promises;
 * added `startAsync` method;
* BackPressure (new package):
 * added `pausable`, `pausableBuffered`, `controlled` methods and `ControlledObservable` interface.