Creating an simple example

A simple example to help many guys approach redux.

- [ ] Reducers tests
- [ ] Thunk middleware tests
- [ ] Injecting repository
- [ ] Using [built_value](https://pub.dev/packages/built_value) and [built_collection](https://pub.dev/packages/built_collection) for:
- are immutable, if the elements/keys/values used are immutable;
- are comparable;
- are hashable;
- use copy-on-write to avoid copying unnecessarily.

I will take care of this task.