class#::AVAILABLE_#{field} shows available flags

Dynamically inject a class constant which returns an array of
available (known) flags. I.e. when using the default name and
extending a model named User with the flags [:flag1, :flag2]

User::AVAILABLE_FLAGS will return [:flag1, :flag2]
Merge branch 'pr/3'

* pr/3:
  class#::AVAILABLE_#{field} shows available flags
remove unneeded commented code