Add token authenticatable model abstraction

**As a** developer 
**In order to** be able use _Simple Token Authentication_ with other models than `User` 
**I want** to be able to specify the _token authenticatable_ model for each _token authentication handler_ I declare 

**Preliminay refactoring**: as a maintainer, in order to make contributions easier, I want the dummy app generation for testing purpose to be easier, and the resulting scenarii to be more readable.