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
Make it a public variable and make it possible to override it from the outside.

Let's say we call it "DOCKER_BIN" (capital letters for variable names.) I think "DOCKER_BIN" is pretty descriptive but you might want to add more info. In any case, document it that it's the path to the "docker" binary, mention that in the boot2docker case it needs to be the one inside the VM.

```shell
if [[ "${DOCKER_BIN+set}" == "set" ]]; then
  echo "Using DOCKER_BIN=${DOCKER_BIN} from the environment."
elif DOCKER_BIN=$(which docker); then
  echo "Setting DOCKER_BIN=${DOCKER_BIN} from local machine."
else
  echo "Could not find a working docker binary and none passed in DOCKER_BIN." >&2
  exit 1
fi
```

Do you think something like that would be reasonable?

If you can detect boot2docker (I think I've seen an is_osx routine around, maybe that's enough?), then you might add another `elif` (before the `$(which docker)` case) defaulting it to `/usr/local/bin/docker` since that's the default in boot2docker, right?

Cheers,
Filipe