Fixes I18n for models with tuple permissions

Fixes an error when `I18n` tries to add its permission to a model with a tuple for permissions. A model's `cls._meta.permissions` attribute can either be a list or a tuple (see https://docs.djangoproject.com/en/1.4/ref/models/options/#permissions).
Merge pull request #16 from khalifenizar/i18n-tuple-permissions

Fixes I18n for models with tuple permissions