[todo-ribs]: proposal for RIBs architecture

Hi, I started working on the [todoapp](https://github.com/aitorvs/android-architecture/tree/todo-ribs) to implement it using [RIBs](https://github.com/uber/RIBs) architecture, as I wanted to get familiar with it and this was the best manner.

Do you see any interest on including this as part of the android-architecture repo, when finished?

Start by reviewing the linked todoapp implementation on the todo-ribs branch and the RIBs architecture referenced in the issue. The issue does not identify files, tests, inclusion criteria, or a definition of done; clarify whether and how the completed sample should be included in the repository.

Written by the indexing model from the issue text.