Fix Namespace in x509_to_cybox.py

`cybox.utils.IDGenerator` expects a `Namespace` and not a `string` : 

```
$ python x509_to_cybox.py  -i example/example_certs.txt -o a.xml
Traceback (most recent call last):
  File "x509_to_cybox.py", line 287, in <module>
    main()    
  File "x509_to_cybox.py", line 272, in main
    observables_list.append(cert_to_cybox(cert_dict))
  File "x509_to_cybox.py", line 37, in cert_to_cybox
    observable_dict = {'id': cybox.utils.IDGenerator('x509_to_cybox').create_id(prefix="observable"), 'object' : x509_obj_dict}
  File "/home/yoyo/.virtualenvs/cybox-tools/src/python-cybox/cybox/utils/idgen.py", line 27, in __init__
    self.namespace = namespace
  File "/home/yoyo/.virtualenvs/cybox-tools/src/python-cybox/cybox/utils/idgen.py", line 38, in namespace
    raise ValueError("Must be a Namespace object")
ValueError: Must be a Namespace object
```