Adds a Procfile that will work with Heroku

Pretty self-explanatory. 

Whoever uses this needs to

1. Create an app with the https://github.com/kr/go-heroku-example buildpack
2. Set `HMAC_KEY` to whatever their key is

Here is an example of it running on Heroku:

![](https://disko-camo.herokuapp.com/5640024f97434098d5afb455c1f1fab08372a66b/687474703a2f2f692e696d6775722e636f6d2f3057676a362e676966)