Merge pull request #68 from taigaio/django1.7-step2-bugs

Fix wrong assignation of invited_by field on  invitation is created.
Add split_by_n function on utils.iterators module.
Add iter_queryset method to utils.iterators module.
Add cached name and size fields to attachment model.
Change the way to generate attachment resource path.
Apply the same url naming of attachments to user photo.
Remove deprecated attachment view responsible of permission checks.
Add adhoc script for migrate the location and description/content fields to new attachments urls.
Improve attachments factories and fix tests related to storage refactor.