Add `EntitiesManager` specs

**As a** developer 
**In order to** improve the  `ActsAsTokenAuthenticatable` module design through refactoring 
**And** to make contribution easier 
**I want** I need its interfaces to be properly tested 
**And** documented (see #104)  

The logic which allow a same _token authentication handler_ to handle token authentication for several _models_ was untested. That logic is based on a representation of such models as _entities_.

**In order to** make easy to test that logic in isolation, **I want** to extract it to an entities manager, **and** I want it to be properly tested and documented.