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.
They are very similar problems. Again, it feels like there are two aspects to it: first is creating an `Observable<Result>` for people to hook into. Second is to figure out how to actually hook it into an `Activity` (or `Fragment`, since there is a `Fragment.onActivityResult()`).

In this case, it seems like it'd be simple to just have a single `Subject` inside of `Activity` or `Fragment` to handle `onActivityResult()`. But the same subclassing problem crops up (yet again)...