Add Kakao backend

[momamene]

Implemented after https://developers.kakao.com/docs/restapi

[omab]

The code looks good except for the failing test, looks like the docs doesn't specify this return format https://github.com/momamene/python-social-auth/blob/9caf545ccb23955d9a2e26c20ac05923aae7a1ba/social/tests/backends/test_kakao.py#L13-L25, instead it should be:

    user_data_body = json.dumps({
        'id': '101010101',
        'properties': {
            'nickname': 'foobar',
            'thumbnail_image': 'http://mud-kage.kakao.co.kr/14/dn/btqbh1AKmRf/ujlHpQhxtMSbhKrBisrxe1/o.jpg',
            'profile_image': 'http://mud-kage.kakao.co.kr/14/dn/btqbjCnl06Q/wbMJSVAUZB7lzSImgGdsoK/o.jpg'
        }
    })

Right? That's what the backend is expecting too. I'm not 100% sure since I can't read the site :)

[momamene]

Thank you for your advice!
I fixed user_data_body and get_user_details of kakao.py.
Now it passes tests.

[omab]

Great, merged!