Support for Docker

This is a Pull Request which adds Dockerfile to project_template.

If you didn't hear about docker, please take a look: https://www.docker.com/whatisdocker/.

To test this PR, docker has to be installed:

    curl -sSL http://get.docker.com/ubuntu/ | sudo sh
    pip install fig
    pip install git+https://github.com/SpisTresci/mezzanine.git@docker_support

After installing docker, fig and mezzanine, user can build docker image and run docker container by:

    mezzanine-project example
    cd example
    fig up

# Example docker image

I have created another repository, with example project generated by `mezzanine-project` from this PR:
https://github.com/SpisTresci/mezzanine-docker-demo

Thanks to this, docker repository with autobuilder was created:

* https://registry.hub.docker.com/u/noisy/mezzanine-docker-demo/

[![nodesource/node](http://dockeri.co/image/noisy/mezzanine-docker-demo)](https://registry.hub.docker.com/u/noisy/mezzanine-docker-demo/)

User can run mezzanine-docker-demo without building own image, by:

    docker run -d --name mezzanine_db postgres
    docker run -P --name mezzanine_web -p 80:80 --link mezzanine_db:db noisy/mezzanine-docker-demo