In cart_modifiers methods ``get_extra_cart_price_field`` and ``get_extra_cart_item_price_field``
accepts the additional field ``state`` to pass customized data from ``Cart.update()``
down to these methods.
Fixes backwards compatibility issues where state was missing
Replaced the state dict by the request object in all cart modifiers
Must also replace ``state`` against ``request`` in ``CartItem.update``
Merge pull request #217 from jrief/cart_modifiers-fix-missing-state

Add state object to cart modifier methods get_extra_cart_price_field and get_extra_cart_item_price_field