Enable getting an i18n error message with key

[ivanxuu]

PROBLEM: Currently the message has to be included in the same
declaration with:

  multilang :title, :required => 2,
    message: 'Insuficientes traducciones'

It would be better to have a default error key in the translations file.

SOLUTION: Add I18n key by default. The developer will need to add
something like this to his translations file.

  errors:
    messages:
      insufficient-translations:
        one: 'We need at least one tranlation.'
        other: 'We need at least %{count} tranlations.'