Enable RSpec for autonomous testing, add configuration

Add RSpec to the default rake task

Run `rake spec` to run all unit tests.
Add ActsAsTokenAuthenticatable specs
Minor refactor make external namespace explicit

Avoids the ActiveRecord Module to be searched into
the SimpleTokenAuthentication::ActsAsTokenAuthenticatable namespace, and
improves the error message when ActiveRecord is not properly required.
Add specs for inherited token authenticatable behaviour
Refactor remove redundant code
Add specs: generated auth. tokens are unique
Add proper tags to public interface specs

Add spec:public rake task
Refactor extract TokenGenerator, invert dependency

The dependency to the Devise token generator is nw encapsulated in
the `ActsAsTokenAuthenticatable#token_generator` method.

The `ActsAsTokenAuthenticatable#generate_authentication_token` method
can be tested through a DummyTokenGenerator.
Merge branch 'add-acts-as-token-authenticatable-specs'