Enable customized Order short name

This pull makes it possible to override short name of an order, used as a payment text, implemented in shop_api.py with a custom property of the Order class.

For example:
```python
class Order(models.Model):
(...)
    @property
    def short_name(self):
        """
        A short name for the merchant order.
        Should be human-readable, as much as possible.

        """
        return self.order_number
(...)
```