Rename script/setup to bin/setup

This will be the Rails 4 recommendation:

https://github.com/rails/rails/commit/009873a

* Include `-e $RACK_ENV` in `Procfile` per Heroku docs.
* Make bin/setup idempotent by only copying `.sample.env` to `.env` if
  `.env` file does not exist yet. Prevent users from accidentally
  overwriting the file, which is not kept in version control.
* Change `sample.env` to a `.sample.env` so it shows less often.