Fix some readme and script bugs in examples (take 2).

so -- `sudo` here is an intersting one.  There are 3 cases here:

1. On the mac with boot2docker, you don't need sudo.  In fact, if you run under sudo things will probably break and it doesn't inherit the `DOCKER_HOST` env variable.
2. On linux where the user isn't a member of the `docker` group you need sudo.  This is the common mode when logging into GCE.
3. On linux where the user *is* a member of the `docker` group -- things work great without sudo.  But sudo doesn't harm things.

With this in mind I'd say perhaps have a test to see if you are on the mac...

```
sudo="sudo"
if [[ "$OSTYPE" == "darwin"* ]]; then
  sudo=""
fi

$sudo docker build...
```