Added ``data`` as optional JSONField to extra price models.

Cart modifiers can add an optional ``data`` field beside ``label`` and ``value`` for both, the ExtraOrderPriceField and the ExtraOrderItemPriceField model.
This extra ``data`` field can contain anything, which is serializable as JSON.

This is based on issue https://github.com/divio/django-shop/pull/139 which however used a TextField for the ``data`` field. Since additional data can have any format, this pull request uses a JSONfield. This avoids additional string-to-object conversions.