Set up deployment with Docker.

This pull request sets up Hubot Control for deployment with Docker.

The following changes are made

* A Dockerfile is created
* Instructions for running with Docker are added to the README
* Add defaults to `config/database.yml` and load settings from environment variables.

I'm currently running this in a production environment and it works well. The only potential "issue" is that the process is run as the root user, which is shown as a warning in the Hubot Control interface, but running processes as the root user in Docker containers is standard practice.