Add entities manager interface specs
Fix error of logic in configuration storage

The configuration of Simple Token Authenication for a given
token authentication handler should be shared between all instances
of that token authentication handler, therefore should not be
stored in instance variables but class variables.

Note: The token authentication handler class definition is unlikely
to be modifed between instances creations, therefore the error would
rarely have observable effects. However, its presence got quite
obvious when defining the `entities_manager` class method.
See the _next_ commit!
Refactor extract EntitiesManager, inject dependency

Add EntitiesManager specs

Does in fact encapsulate the internal dependency on Entity,
but the main point was to make the entities managment testable.
Refactor only load dependencies where needed
Merge branch 'add-entities-manager-specs'