Use docker

Docker support for development.

* added Dockerfile to support the creation of Docker containers
* updated README with information on how to setup a Docker container
When you run boot2docker up, it outputs some suggested environment variables, e.g.

 To connect the Docker client to the Docker daemon, please set:
     export DOCKER_HOST=tcp://192.168.59.103:2376
     export DOCKER_CERT_PATH=/Users/mcote/.boot2docker/certs/boot2docker-vm
     export DOCKER_TLS_VERIFY=1

We could automatically support OS X by first looking for the DOCKER_HOST environment variable before falling back to localhost. Then your instructions above can be just telling the user to set the appropriate environment variables as per the output of "boot2docker ip".