API stubs for CSV downloads.

The [README](https://github.com/reddmetrics/forma-api/blob/develop/README.md) has info on spinning up the dev environment. Once the dev app server is running, you can hit API stubs like:

http://localhost:8080/api/prob/world.csv

The full [API spec](https://docs.google.com/document/d/1xxcRcOtH62Z65E6tj9IkCZ0ySUv_VBBLRoRCv_ljbIg/edit#) is here.