Drf-cached-instances

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* Drop support for Django 1.7, Python 2.6
* Expand tests to Django 1.10, Django REST Framework 3.4, Python 3.5

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* Add serializer for datetime.timedelta, to support Django 1.8's
DurationField (creynold)
* Convert string representations of datetimes, dates, and timedeltas
* Expand tests to Django 1.9 beta 1 and Django REST Framework 3.3

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* Remove deprecation warning in Django 1.8
* Expand tests to Django master and Django REST Framework 3.2
* Fix invalid mock.patch tests that break under mock 1.3.0
* Documentation updates and fixes

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* Move get_object_or_404 to mixin method, to allow easier extending.

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* Tested with Django 1.8
* Tested with Django REST Framework 2.4, 3.0, and 3.1
* CachedModel now supports .pk attribute as an alias, usually to the .id
field. DRF 3 uses .pk to determine if a model is saved to database, and
returns empty relation data for unsaved fields.
* cache.delete_all_versions() will delete all cached instances of a model and
PK. This is useful when changes are made outside of normal requests, such as
during a data migration.

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* Add ``update_only`` option to ``cache.update_instance``, to support eventual
consistency for cold caches.