Make it easier to whenever-ize an application

[floehopper]

Harmonia, Sauron, and Heylist use whenever in conjunction with recap. It would be nice to make this easier. I'm guessing the right way would be to contribute to the whenever gem...?

Also when I did this for Heylist, I had to comment out the clear_crontab hook when I ran cap deploy for the first time after adding whenever to the Gemfile in order to avoid an error.