Allow to specify custom storage for enum class

[estum]

Allow to specify custom storage for enum class via:

• self.storage= method inside enum classes:

  class Sex < ActiveEnum::Base
    self.storage = :i18n
  end

• :storage option for enumerate method with block:

  class Person < ActiveRecord::Base
    enumerate :sex, storage: :i18n do
      # ...
    end
  end