Make dev/staging work on Heroku

I've been experimenting with deploying to heroku, and would like to switch dev.growstuff.org over to run there instead of on the current server.

The only change here is to config/environments/staging.rb to send email via mandrill.  Everything else is just in heroku settings which I've documented here: http://wiki.growstuff.org/index.php/Heroku_notes