Minor refactor reword the auth. requirement feature (again!)
Add Cucumber step for multi-model Devise setup
Ensure several models can act as token authenticatable simultaneously

Add Cucumber spec to prepare two authenticate_model! methods for RSpec
Add (passing) regression test with two token authenticatable models

Note: the RSpec spec examples are run in a random order, that's why
the Cucumber scenarii can't rely on the output of several examples
at once (see multiple `And the output should match:` steps).
Add executable documentation entry in README

Add Rake task to generate doc/features.html
Add up-to-date features documentation (HTML format)