Currently, handle_payment_event defaults lines and line_quantities to None,
however, calling it with these defaults will result in an exception being
raised in validate_payment_event because it expects iterables.

This commit also adds tests to cover validate_payment_event.

As they save stuff...

We now check in the OrderCreator for this attribute and only allocate if
it is true.

Fixes #671

Fixes #376

It didn't exist when I wrote patch_settings but now there's not point
having two versions of the same thing.

The OrderDiscount did not store (a copy) of the offer
name within a separate field which causes an issue when the
offer is deleted. E.g. it breaks the order history view for
the customer because no offer data is available.

I have added a new field to OrderDiscount that is automatically
populated from the offer.name field. This works in the same way
as voucher_code does in the same model.

I also found two issues with the voucher implementation within
the same model that are fixed now.

Fixes #330

* replaces all occurences of ``datetime.now()`` with
    ``django.utils.timezone.now()`` which returns aware or naive
    datetimes based on ``USE_TZ``
* increases ``South`` version to 0.7.6 in dependencies to make
    aware datetimes work in migrations.
* Oscar's sandbox is now configured to use aware timezones by
    default