Provide access to the local port used by a connected UdpSocket

[totalgee]

• add new method LocalEndpointForConnectedRemoteEndpoint(), and modify LocalEndpointFor() to work when the socket is connected.
• lets a UdpTransmitSocket find out which local port it's using to send, allowing the possibility to open a receiver socket on that port and thereby send and receive on the same local port. This makes things easy and symmetrical for client/server uses; the server can simply reply to the ports from which it receives client messages.
• includes code from @RossBencina, after a discussion in August 2014.

[totalgee]

More info about this here: https://code.google.com/p/oscpack/issues/detail?id=18