Use RACK_ENV from .env in development

* Include `-e $RACK_ENV` in `Procfile` per Heroku docs.
* Remove unnecessary `sample.env` from generated Rails app repo.
* Make script/setup idempotent by only adding `RACK_ENV` environment
  variable to `.env` if not in file.
Rename script/setup to bin/setup

This will be the Rails 4 recommendation.
Check existence of .env file

Focus on preventing users from accidentally overwriting the file,
which is not kept in version control.
Make sample.env a dotfile