Added suport to "connect back" option

The _advertise_ option should be improved.

If the initial server the agent connects to, could get the peer's ip from the connection instead of "trusting" on the advertising option. This can be achieved by allowing a special value to the advertising option (something like "connect-back") or adding another option that overrides the advertising value.

This is very handy for ephemeral servers running on (for example) AWS auto scaling groups.