Added Observable.zip() which was missing.

https://github.com/Reactive-Extensions/RxJS/blob/master/doc/api/core/observable.md#rxobservablezipargs

rx.js/rx.js.d.ts

@@ -406,6 +406,7 @@ declare module Rx {
 		merge<T>(scheduler: IScheduler, sources: IObservable<T>[]): IObservable<T>;
 		onErrorResumeNext<T>(...sources: IObservable<T>[]): IObservable<T>;
 		onErrorResumeNext<T>(sources: IObservable<T>[]): IObservable<T>;
+		zip<TResult>(...soucesAndResultSelector: any[]): IObservable<TResult>;
 	}
 
 	var Observable: Observable;
@@ -456,4 +457,4 @@ declare module Rx {
 		onError(exception: any): void;
 		onCompleted(): void;
 	}
-}
+}