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.

Tengo que revisar lo de VIPER. Estuve es una charla en Codemotion y la verdad es que me pareció muy interesante..