Added new attribute to Bundle to contain related objects to be saved in ModelResource.save_related
Due to a bug in Django (ticket https://code.djangoproject.com/ticket/18153) and the corresponding patch (https://github.com/django/django/commit/3190abcd75b1fcd660353da4001885ef82cbc596), tests were failing with Django 1.5 (tests/validation/). This commit modifies ModelResource so that related resources no longer rely on this incorrect Django behavior by storing the objects to be saved in save_related (which is called after authentication/authorization checks).