Add "quiet" option and its effect on `ip` subcommand

I was dissatisfied with current output of `boot2docker ip` as I find the explanatory text a little un-UNIX-ey.  However I realize it is important for usability.  This PR adds `-q` / `--quiet` option so that it will just spit out the IP and nothing else, this enhances scriptability and usage in case like:

```sh
curl $(boot2docker ip -q):5000/users/sven
```