Listening for Git Repo Changes

I want to write an app that given a user's GitHub account information and a repo will be notified whenever the repo is pushed to. I know I could basically save the state of the repo and poll it for changes periodically but I was wondering if there is a way to do this with a push architecture and if so how to go about it. Thanks for any help!

EDIT - I know I can probably doing this like Heroku does it by having them push to a remote server, but the ideal functionality is to know when they push to Github itself.