Minor add EntitiesManager#entities
Minor refactor move spec helpers
Add TokenAuthenticationHandler public interface specs
Refactor extract TokenAuthenticationHandler

Refactor rename `define_acts_as_token_authentication_helpers_for`
to `define_token_authentication_helpers_for`

Refactor extract method `set_authentication_hooks`

The module specs are largely copied from the specs of
ActsAsTokenAuthenticationHandler. In order to keep the diffs
readable, I'll clean those in the next two commits.
Add updated ActsAsTokenAuthenticationHandler specs
Remove obsolete ActsAsTokenAuthenticationHandler specs
Minor refactor DRY ActsAsTokenAuthenticationHandler specs
Revert "Minor add EntitiesManager#entities"

This reverts commit ab9c3f89d2b18d0b592d09e79f17fa95bf3831a6.
Minor add spec helper

Minor refactor rename spec helper
Refactor DRY ActsAsTokenAuthenticationHandler
Refactor remove ActsAsTokenAuthenticationHandlerMethods

The behaviour that module used to add is part of
acting as a token authentication handler.
Keep private the token authentication handler interface

It makes sense for a token auhentication handler to be some kind
of authentication handler; one could therefore expect it to
implement the 'authentication handler' interface.

Implementing the authentication logic in a TokenAuthenticationHandler
class (versus module), would IMO make sense, but would require a major
renaming of public methods (starting by acts_as_token_authentication_hanlder).

Such a renaming could very probably lead to a better public interface
(e.g. setting `before_action :authenticate_user_from_token!` in
controllers, instead of the somewhat obscure token authentication
handling thing); however, it would _a priori_ require a major version
jump, which I don't want to perform now.

Keeping the interface private allows to decide later, and the main feature
of this branch --the interface specs-- are as useful being private
as being public.
Merge branch 'add-token-authentication-handler-specs'