Heroku deployment improvements

This PR refactors/improves the Heroku deployment process. Major tasks:

* [x] Moves the script to a rake task
* [x] Removes all direct invocations of the system, preferring gem use
* [x] Uses a Heroku API key rather than username/password combo for improved security
* [x] Option to provide sensitive credentials at runtime for improved security

Current issues:

* Heroku deploy output is suppressed until deploy is complete (I don't think there's a way around this)

@herestomwiththeweather please take a look at the new approach and give a thumbs up/thumbs down as necessary. I'll be fleshing out the update script and fixing any issues you raise later this week, at which point I'll update the PR for you to merge.