PEP8 openbazaar.py through ws.py

PEP8 clean up 3 of 3.  Orders.py has some very deeply nested dictionaries which necessitate long lines.  This situation may suggest minor future refactoring of the affected methods.
An empty dict evaluates to `False` inside a boolean context. A `None` value also evaluates to `False`, so here's a succinct way to write the entire conditional:
```python
offer_images = offer_data_json['Contract'].get('item_images')
order['item_image'] = offer_images or "img/no-photo.png"
```