Make MadProps support DRF 3.0

[ekoval]

This is actually a new implementation. DRF changed so much that it's impossible to just "patch" existing implementation a bit.

How it is done:

The key idea of whole change is:

• JSON -> object: take {<prop_name>: <prop_value>...}, convert it to format acceptable by DRF serializers, then call DRF standard code. (to_internal_value method and friends).
• object -> JSON: call DRF, then convert result to a {<prop_name>: <prop_value>...} form (to_representation and satellites).
• Since we have completely different save logic - .save() method is re-written from the scratch.
• Unfortunately,DRF 3.0 doesn't support nested writable serializers, so we had to introduce a base class for property owners. That class overrides .update() (probably we need to add .create() as well), and manually saves properties after the parent object is saved.

  Tests are coming soon.

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[zoidyzoidzoid]

I think you need to merge in up-to-date master.

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[ekoval]

@zoidbergwill : merged.

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[adrianmoisey]

Merge conflict

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[RayeN]

:+1: Comments are minor and style related. Well done!!!

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[RayeN]

:+1: