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
BTW, there's a `2> /dev/null` on one command in `clean.sh` just to make sure the build image isn't lingering around. Docker should be able to take care of removing it (--rm --force-rm), but the script still attempts as docker might fail to do its job properly or a manual build attempt might have one lingering around.