GetName method undefined in pbtxt2dot.py

In pbtxt2dot.py, the Sort method seems to be calling an undefined GetName method. I had to add the method to get it working.

``` python
def Sort(model):
  def GetName(edge):
    return '%s:%s' % (edge.source, edge.dest)

  S = []
  L = []
```

Please correct me if I am wrong. Thanks.
