Create response on json for cities and countries

 - Removes foreign key on cities, for conflict on id.
 - Change wrong field name contry for country on table countries.

Signed: Uriel Molina<uriel.molina@diverza.com>
Add pollutants_agents service.

Signed: Uriel Molina<uriel.molina@diverza.com>
Add stations service

Signed: Uriel Molina<uriel.molina@diverza.com>