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.
Oh, one complaint from me: the `git` or `basename` above can fail, and we should check for that. `set -e` might be the best way to do that here, given the complexity of the code.