Optionally listen on 0.0.0.0 in standalone mode

I found that, for development, I often have to make the standalone
server listen on 0.0.0.0 (instead of 127.0.0.1, which is the default) in
order to test the system with multiple agents.
Merge pull request #242 from guidow/dev_listen_on_wildcard

Optionally listen on 0.0.0.0 in standalone mode