docker: add cluster bootstrap and doc

This bootstrap a local kubernetes cluster by running the `kubelet` as a bare`docker` container, and passing it a pod manifest with the cluster components.

Changes:
- add getting started guide for `docker` locally
- modify `build/` to allow boot2docker on all platform
- add `kubernetes-bootstrap` image
- embed cluster-pod manifest in bootstrap image

You should be able to try this at home with:
```
docker run -d -p 10250:10250 -v /var/run/docker.sock:/var/run/docker.sock proppy/kubestrap
export KUBERNETES_MASTER=$DOCKER_HOST_IP:8080
kubecfg list pods
kubecfg -p 8181:80 run nginx 1 kube-nginx
curl $DOCKER_HOST_IP:8181
```

Ack.

Make senses to build form source as this flow is positioned as an alternative to `local-up-cluster.sh`.

Later when we start pushing run images to the hub, we can have another guide more end-user centric.