Improved project structure following teacher's advice. New package domain
now contains all the intelligence of the application and data. Controllers
are just for filling view with data coming from domain.
Domain is composed as follow:
- helpers: tools to do recurrent tasks, in different classes, like calling
  web service.
- model: contains object representing the data (for example we'll probably
  have a class called Trip, or User... That we will fill with data
  retrieved from the webService)
- services: intelligence of the app, containing classes to interprate results coming from server