Rewrite of the Guestbook example in Go

This is a Go version of the Guestbook example. The readme has been ported over from the existing one.

#1271
We don't expect to update this image often, but it would be nice if the docker registry user was parameterized to make it easy to change so that other folks can create a new version. It's fine to leave your username as the default as long as it's parameterized and can easily be overridden (is there a common env variable like DOCKER_USER that we could check?).