[WIP] OS X install support

- [x] Install `boot2docker` when running script on OS X
- [ ] Perform `boot2docker` initialisation in `bootstrap.sh` to get `docker` up and running (https://docs.docker.com/installation/mac/)
- [ ] Check setup is persisted after reboot
- [ ] Maybe also install `wget` at the start of `bootstrap.sh` for OS X, as it doesn't ship with it
Was just wondering if we could do it without if-branching that's all. But see my reply in the main PR thread; I think the if-branch is fine if it doesn't get too complex.