Add `bin/deploy` script

* Help avoid forgetting to run `rake db:migrate`.
* Help avoid forgetting to run `restart` after migrate.
* Allow each app to edit these deploy instructions to fit their
 environments' needs (such as `run rake purge` to clear a cache)
 but maintain the same `./bin/deploy` interface and convention
 that is consistent with our many Middleman and Rails apps.
@georgebrock I think the only ways the script can fail now are on `git` and `heroku` commands. I'm thinking the earlier commit in this branch that removes `set -e` and goes with `&&` is still preferable. Thoughts on the squashed commit (and its commit message) 2f0184d, @georgebrock @mike-burns @pbrisbin?