[ISSUE-95] Send replies from the right source IP address

Fixes #95

I didn't test this enough, but lets see what you have to say about it :)
I tried to keep it as simple as possible.

This has to be tested with IPv6. I guess that IPv6 is not supported yet (shouldn't be too hard to add IPv6 support though), but first lets make sure it doesn't break it.
I also still believe it is possible to make a switch here, revert to srv.serveUDP(l) when *not* listening on 0.0.0.0 or ::1 and calling NewUDPConn() and friends otherwise. This can then be a boolean in the server struct, to switch this on/off. If this is working for enough people it could switched on for everyone.