Dockerizing the app: distributing this application with a Dockerfile

This commit dockerizes this application and configures the config.json
to properly use the settings from the environment (as advocated by
12factor).

docker run -t -i -p 8181:80 --link npm-registry:couchdb --link redis:redis \
    --link npm-registry:private_npm marcellodesales/browse-npm

- couchdb: a running couchdb container running with the default ports
- redis: a running redis container running with the default ports
- private_npm: a running private npm container.

This uses the same npm-registry container based with kappa.

*	new file:   Dockerfile
- New Dockerfile based on Ubuntu 14, that builds the current application.

*	modified:   config.json
- Adjusting the configuration to properly load the configuration from the
  environment variable.

*	modified:   README.md
- Updating the docs with instructions on how to run.