Adds kube2sky public IP mode

As per our conversation over IRC today, I played around with adding a mode to kube2sky where it would register only the public IP addresses of services. This is a rough first pass, it works, but has some rough edges. Would love to hear your thoughts.

Add -type=public to the kube2sky command and it will create entries in DNS for every public IP address a service has. It uses the "{1..N}.service.namespace.domain" convention suggested by the skydns docs for multiple IP addresses behind a single name. Not sure if that's appropriate here.