Support for callables on cart product lookup setting

[lazybird]

The default queryset manager is used and all products are retrieved. You can filter products by defining some lookup parameters in CART_PRODUCT_LOOKUP setting.

The CART_PRODUCT_LOOKUP should support passing in callables, making this possible:

CART_PRODUCT_LOOKUP = {
     'status: 'A',
     'date_valid_from__gte': datetime.datetime.now
}