text: Title: Create JSONField which wraps implementation of django.contrib.postgres and jsonfield
  Body: **djangoSHOP** currently uses the `JSONField` from https://pypi.python.org/pypi/django-jsonfield which is portable and the only available choice on Django-1.8.
  
  Now that Django ships with its own `JSONField`, we should use that. Unfortunately it is specific to Postgres only, hence we need a mechanism which wraps the `JSONField` and only uses the Postgres specific one, if we know for sure, that the database runs on Postgres, otherwise it uses the other one.
  
  Since all occurrences of `JSONField` have  `default=dict`, this default shall be integrated into that wrapping field. This field shall be implemented in `shop.models.fields.py`.
  