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
You can actually build guestbook without needing to ssh into boot2docker vm since the docker on host is only a client. The volume mounting takes place in the vm where the docker daemon resides. Condition is that the result of `which docker` is the same on both host and the boot2docker vm, which in my case is `/usr/local/bin/docker`. Making exceptional handling of boot2docker would be too much IMO. I'm adding this information to the guestbook src's readme file instead.