Simplify deployment procedure to Heroku

Having to configure the separate account with MongoLab is somewhat tedious and unnecessary if you are deploying to Heroku, as there exist MongoLab & MongoHQ addons which simplify the process quite a bit (no signups!).   These are very minor changes that just implement the plugins, but the net effect is that someone can fork hackathon-starter then immediately deploy it to Heroku and have it work.  Having done two hackathons already using this repo, I can say that having hackathon-starter live within 20 seconds is a very inspiring way to start a hackathon =)

I was considering editing the docs to place the deployment details toward the front of the readme to show how easy it is to do. I figured I would wait to see your response first -- let me know!