Refactor treat ActiveRecordAdapter as a generic adapter

This could be seen as supporting multiple ORM/ODM/OxM adapters.
Add :adapters option

Keep loading the ActiveRecordAdapter by default because
the Cucumber features depend on it. Even if the Cucumber steps
could be modified to handle that dependency, I prefer not to
do it here nor now.
Add Adapter module to make easy adapters definition

Any class whcih extends SimpleTokenAuthentication::Adapter
implements the adapter interface. Its required methods must
be overridden, and conveniently raise a NotImplementedError
when the override is missing.