Add Heroku driver

Because app engine folks don't allow streaming and limit max response content
length to some unreasonably small amount which can be an issue for long reads.

To run locally on Heroku toolbelt use: foreman start

To run on the server, import env vars to server config with:
    heroku config:set HEROKU=1
    heroku plugins:install git://github.com/ddollar/heroku-config.git
    heroku config:push