Feature: better Heroku support - move configuration to environment variables & Procfile

Two changes/additions:
1. Use of Procfile (via [Honcho](https://github.com/nickstenning/honcho); Python port of Foreman). The reason we have both Procfile and Honchofile is to be able to run both the workers and the web server in a single Heroku dyno.
2. Pass configuration via environment variables instead of updating `settings.py`. If you're using Honcho to run the project, note that it supports reading environment variables from a `.env` file (see the `.env.example` for format).

This version can be run "out of the box" on Heroku. All you need is to download the latest release, push to Heroku and update the environment variables.