Refactor createStore to remove StateStream usage

[agentk]

The RxSwift Store now uses the original code again:

subjectDispatcher
.scan(initalState, accumulator: reducer)
.startWith(initalState)
.subscribe(stateSubject)

The setupStoreBacking function returns a tuple of the dispatch sink, state subject and createStore function. It can be used to expose the underlying dispatch and subject for direct usage inside an app.

It's useful for subscribing to nested properties. Ie:

let factory = setupStoreBacking()

let store = applyMiddleware([
    observableMiddleware
    ])(factory.createStore)(reducer, state)

factory.subject.map { $0.auth.loggedIn && $0.auth.tokenExpired }.onNext { tokenExpired in
    // Do something when a state is emitted where the user is logged in and the token is expired
}