Split configuration into environment-specific

In order to deploy to production we need to do some things like static
files, SECRET_KEY, turn off DEBUG etc.

Also 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/
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
Add to README with how to set up Heroku app

There's quite a lot of stuff you need to do when you create a new app on
heroku, so it's recorded here.