Drop support for Django < 1.8 and related cleanups

Added DummySerializer to use when values to get/set are already seria…

The need arose when I wanted to cache serialized output of django-tastypie list endpoints. The output was json already, so it makes no sense to re-serialize it with pickle parser or json parser.

Use default timeout in set_many.

get_many and set_many should work the same way as in other cache backends

Instead of raising an exception it should just return without doing anything.

Instead of raising an exception it should return an empty dict

This is the same behaviour that the default Django cache backends have.

Bugfix: set_many does not set key version

Use line blocks to preserve AUTHORS formatting.

Use line blocks to make sure that each entry in AUTHORS.rst appears on
its own line, as opposed to squished up in one paragraph.

Jf/delete many

RedisDummyCache