Adding a ReactiveNavigator to handle toActivityForResult in an Observable way

The aim of this component is to handle the navigation to an Activity for result using an Observable and thus avoiding the need to check for result/response code and all the boilerplate code related.
I added that to follow the convention on this repo at the time (cf https://github.com/ReactiveX/RxAndroid/blob/e42bb5a80b7e38fffa3c9e79f4e90f6b7f8fad75/src/main/java/rx/android/concurrency/AndroidSchedulers.java). I didn't realised it had changed, I'll remove those now.