implement optional interface for socket

This provides support for an ipv4 IP address to be provided to address the usecase of listening on specific interfaces and is optional, defaulting to 0.0.0.0.
update readme and example for new option

server.listen updated with changes from a7c859d
fixed addr error when using callback
fix addr value with multiline if
Merge pull request #35 from jawsome/bind-interface

Bind interface