fun <A, B> Observable<A>.foldToObservableValue(initial: B, folderFun: (A, B) -> B): ObservableValue<B>
foldToObservableValue takes an rx.Observable stream and creates an ObservableValue out of it.
initial - The initial value of the returned observable.
folderFun - The transformation function to be called on the observable value when a new element is emitted on
the stream.