STUN check fixes. Re Issue #816

 - Now STUN server is returning what should be the outer port on which
   it could listen to the OpenBazaar node (a random one)
 - The --server-port argument is ignored unless --disable-stun-check is passed.

The reason being default behavior is configured so that OpenBazaar will try to establish all its connections without user intervention.
A user that knows on what port they want OpenBazaar to run and who is manually forwarding ports should then read the help and understand the port is being ignored due to the default STUN check openbazaar attempts.

  Try setting a manual port, the one you have set on your router.
  `openbazaar --disable-stun-check --server-port <your_port> -d --log-level 10 start`

  Try letting openbazaar do its work by looking up a valid incoming server port via STUN, and to automatically do the UPnP port mapping on such port.
   `openbazaar start`