Added server SendTo method

[gpayer]

This pull request allows bidirectional communication. To enabled this a SendTo (analogue to PacketConn.WriteTo) was added to the Server struct. Now it is possible to both send messages to a server and receive its answers on the same port.

Further changes:

• Store current connection in Server and use it. Therefor a SetConnection method was added as well and net.PacketConn parameters were removed.
• Added Server.Listen(): enable the listening port, but do not start the Serve() loop yet. Useful if you send a message before listening for responses.
• Packet.SenderAddress(): returns sender address (or nil) for the current message or bundle. This is necessary so a server can send responses to the correct address.