Django-x509

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- [models] Set ``validity_start`` to 1 day before the current date (at
00:00)

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- [django] added support for django 1.11

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- [models] improved reusability by providing abstract models
- [admin] improved reusability by providing abstract admin classes
- [views] provided a base view that can be reused by third party apps
- [docs] documented how to extend models and admin
- [docs] documented hard dependencies

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- [model] avoid import error if any imported field is ``NULL``
- [admin] added ``serial_number`` to ``list_display`` in ``Cert`` admin
- [model] avoid exception if x509 subject attributes are empty

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- improved general ``verbose_name`` of the app
- added official compatibility with django 1.10
- [admin] show link to CA in cert admin
- [admin] added ``key_length`` and ``digest`` to available filters

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- fixed x509 certificate version
- renamed ``public_key`` field to more appropiate ``certificate``
- show x509 text dump in admin when editing objects