RxScala zip

[headinthebox]

Why keep this private? I want it all the time. Perhaps we can call it zipWith to match the Haskell name to avoid bikeshedding that it is not in Scala (it should).

private def zip[U, R](that: Observable[U], selector: (T,U) => R): Observable[R] = {
    toScalaObservable[R](rx.Observable.zip[T, U, R](this.asJavaObservable, that.asJavaObservable, selector))
  }

[benjchristensen]

@zsxwing and @headinthebox Want to discuss and finish this one?

[zsxwing]

@samuelgruetter explained the reason in this commit: 24e0baf

Scala collections do not have such a function, and we want to be
as close to Scala collections as possible

[headinthebox]

I want it in, we can have more operations than other collections have. In fact we should have the union of Scala collections and Rx operators.

(Scala does not have combineLatest either).