Android - Adding a new RetainedFragment example

I wanted to augment the current ```RetainedFragment``` example class to address some scenarios I have run into using the patterns defined in the current example.  

I was hoping to get some feedback on a more complete scenario I have been working on which is the following.

On a typical login screen when the user taps a button we want to fire off our network request in an observable. We want to show a progress dialog and then show either success or an error dialog when it fails. We want to gracefully handle rotation and app switching in case a user does something else after firing off the login call. When the user returns we should be able to tell them there was an error or continue forward. 

I tried to address this scenario above but specifically I wanted to try and complete the following.

* Get all the benefits from the current example - i.e. rotation support
* Make sure the callbacks are not triggered while the app is not in focus
* Make the example be triggered by a user action - i.e. a user tapping a button

Hoping to get some feedback on the example and see if there are any issues with the sample implementation. 

Any issues you see here @mttkay or other android RX users?

Thanks