Django-ar-organizations

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* Bugfixes for deleting organization users
* Removes additional `auth.User` references in app code

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* Bug fix for user model import

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* Manage organization models with South
* Added configurable context variable names to view mixins
* Added a base backend class which the Invitation and Registration backends extend
* Lengthed Organization name and slug fields
* Makes mixin model classes configurable
* Improved admin display
* Removes initial passwords

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* Changes SlugField to an AutoSlugField from django-extensions
* Base models on TimeStampedModel from django-extensions
* ForeignKey to user model based on configurable user selection

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* Added registration backend
* Various bug fixes

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* Add RequestContext to default invitation registration view
* Fix invitations