get_url() for PTOFS

```
>>> p.get_url("test", "somefile")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File ".../venv/lib/python2.7/site-packages/ofs/local/pairtreestore.py", line 119, in get_url
    return self._store.get_url(bucket, label)
AttributeError: 'PairtreeStorageClient' object has no attribute 'get_url'
```

I can't seem to use PTOFS.get_url()
