Add `TokenAuthenticationHandler` specs

**As a** developer 
**In order to** be able to test the authentication logic in isolation 
**And** to make contributions easier with a tests suite which is easy to extend 
**I want** the token authentication logic to move out of `ActsAsTokenAuthenticationHandler` 
**And** to move to its own `TokenAuthenticationHandler` module 
**And** the interfaces of both modules to be properly tested 
**And** documented (see #104)