feat(operators): add window operators (plus some name bike-shedding?)

[benlesh]

The following represent all overrides of the original RxJS 2/3 window:

• window<T>(closingNotifier: Observable<any>) => Observable<Observable<T>>
• windowWhen<T>(closingNotifierFactory: () => Observable<any>) => Observable<Observable<T>>
• windowToggle<T, O>(openings: Observable<O>, closingSelector?: (openValue: O) => Observable<any>) => Observable<Observable<T>>

Plus the additional windowWithCount and windowWithTime:

• windowCount<T>(count: number, skip?: number) => Observable<Observable<T>>
• windowTime<T>(timeSpan: number, timeShift?: number, scheduler?: Scheduler) => Observable<Observable<T>>

[benlesh]

... buffer would face similar renamings.

Thoughts? Objections?