Models + SQLite

This PR contains:

- Definition of the models (1st draft)
    - To take a look at the Entity–Relationship Model: [Entity–Relationship Model gDoc](https://docs.google.com/drawings/d/1OwqbfFlFCeVXoyn874ne3d_TTE421CKfQZkEK0j4RuQ/edit?usp=sharing).

- Scripts to install *sqlite3* and set up the DB