add circle.yml for circleci

[collingreen]

Example without servers (needs template):

machine:
  environment:
    DEBUG: true


## Customize dependencies
dependencies:
  override:
    - pip install -r reqs/dev.txt


## Customize deployment commands
deployment:
  staging:
    branch: master
    commands:
      - git push git@heroku.com:{{app_name}}.git $CIRCLE_SHA1:master
      - heroku run python manage.py migrate --app {{app_name}}