Add dill serializer

[svenauhagen]

Hi,

this is a dill serializer.
I do need it because I use django model utils to track fields.
That plugin does not work with pickle but dill works on serializing models with it:

jazzband/django-model-utils#83

Best and thanks
Sven

[eyesee1]

dill needs to be added to requirements.txt

[svenauhagen]

done

[jdufresne]

I think it would be good to add tests similar to the other serializers.

[niwinz]

Tests are mandatory for new features.

[svenauhagen]

Hi I added some test for dill.
Let me know if that is the correct way of adding the tests for the project.

[peterbe]

If someone knows they want django-redis with dill they'd install it with pip install "django-redis[dill]". To make that work you'd just add to setup.py...:

  extras_require={
      'dill': ['dill'],
  },

[peterbe]

Can this PR also come with a change to the docs?