Configure for deployment to Heroku

- Add to README with how to set up Heroku app
- Configure WSGI server for Heroku
- Add Procfile for gunicorn
- serve static files through WSGI
- add dj-static to production requirements
- set Python runtime to 2.7.7
- Split configuration into environment-specific
- Add EMERGENCY_DEBUG environment variable so that you can turn on
debug in Staging if absolutely required.

See https://docs.djangoproject.com/en/dev/howto/deployment/checklist/

https://www.pivotaltracker.com/story/show/73700008

[Delivers #73700008]