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.
This is still wrong. `RESULT_OK` and `RESULT_CANCELED` are the only two defined codes but the value can be user defined and mean whatever they want. We can't make assumptions like this.