Add failing test for ApiAdmin model
Move API to `acts_as_token_authentication_for Model`

Note: that the 3 features are failing because of `acts_as_token_authentication_for`
being undefined. This temporary regression was expected during the method
signature change.
Support generic authenticatable models

 Token authenticatable behaviour should be applicable to any model as
 long as you added Devise to it.
Fix authenticate_entity! method name for MultiWord models