STREAM event can have a zero port

Now that there is some event handling, I've begun trying to develop against Stem and I found this crash:

  File "/usr/lib64/python2.7/site-packages/stem/response/events.py", line 55, in _parse_message
    self._parse()
  File "/usr/lib64/python2.7/site-packages/stem/response/events.py", line 459, in _parse
    raise stem.ProtocolError("Source location's port is invalid: %s" % self)
ProtocolError: Source location's port is invalid: STREAM 14 NEW 0 
176.28.51.238.$649F2D0ACF418F7CFC6539AB2257EB2D5297BAFA.exit:443 
SOURCE_ADDR=(Tor_internal):0 PURPOSE=DIR_FETCH

StreamEvent should use is_valid_port(port, allow-zero=True) due to DIR_FETCH.

Trac:
Username: robinson