Move script/setup to bin/setup

* `bin/setup` is more in line with Rails 4.0 conventions.
* Include `-e $RACK_ENV` in `Procfile` per Heroku docs.
* Make bin/setup idempotent by only copying `.sample.env` to `.env`
  `.env` if file does not exist yet.
* Change `sample.env` to a `.sample.env` so it shows less often.