Encapsulate dependencies on ActiveRecord (refactoring)

**As a** developer 
**In order to** make dependencies transparent 
**And** to make the code testable in spite of them (see #104)
**I want** all dependencies to be properly encapsulated 
**And** I want them to be injected in the methods that use them 

Now the dependencies on Devise are under control (see #112), let do the same with those on ActiveRecord.