Fix error in bin/setup when missing `foreman`

Back ticks are interpreted when in double quotes:

   $ bash -c 'echo "Missing `missing-command`"'
   bash: missing-command: command not found
   Missing
Re-structure bin/setup to avoid run-on commands
Replace .env setup with non-destructive version

This will only add new entries from .env.sample, leaving existing values intact.

Furthermore, it will allow you to define generators for (e.g. random) values:

    # The following in `.env.sample`...
    SECRET_KEY=$(ruby -rsecurerandom -e 'print SecureRandom.hex(32)')

    # ...might result in the following in `.env`:
    SECRET_KEY=d331561f07ab612c8f8e8f01c688afc03eeac2215ff69f62173543076fa757ff
Merge pull request #70 from bjeanes/setup-improvements

bin/setup improvements