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.
Merge pull request #19 from sealevelresearch/configure-for-heroku

Configure for deployment to Heroku
Script to deploy staging/production to Heroku

This allows us to deploy to staging or production in a totally
consistent way. The workflow between the two is slightly different:

- Production deployment runs a backup before modifying the database.
- Staging deployment copies in the production database before modifying
  the database.

This ensures that staging is an accurate representation of production
for testing eg database migrations, and that production hopefully
doesn't get destroyed on a borked migration.

https://www.pivotaltracker.com/story/show/73700144