Ready to review.

I've tried to explain most of the code in the code.

I think that it covers more or less all requirements, But due to the lack of time, may be I've taken some "licenses".

TODO list:
- Better error handling
- Use pagination from the API to load more pictures.

Thanks for your time.

En mi opinion, la creacion de un contexto hijo del principal la hubiera incluido en la clase RNKDataBaseEngine, asi tendria mas coherencia y no romperiamos single responsibility en este metodo.