ORPort descriptor downloading support

DescriptorDownloader support for downloading from relay ORPorts. To do this
simply provide an ORPort as the endpoint...

  reply = stem.descriptor.remote.get_server_descriptors(
    endpoint = stem.ORPort('12.34.56.78', 90),
  )

Like the stem.client module on which this is based this is still very much
alpha functionality. No doubt as we use this in the wild we'll discover rough
edges. That said, in practice seems to work.

Originally we planned for this to be part of Dave's SoP project, but I need
this capability for something of my own (health check for my tor relay). No
matter - just means Dave has more time for his other project tasks. :P