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.
Simplify:

1. `len(X) == 0` amounts to `not X`

1. `X not in Y or not Y[X]` amounts to `not Y.get(X)` because the default `get` value is `None`, which evaluates to `False`. 

```python
if not os.environ.get('DYLD_LIBRARY_PATH'):
...
```