Fix deletion of STI temp dir on exit

Code cleanup/refactoring.

Change call from os.Remove to os.RemoveAll to remove the temp dir and
its contents.

Only chown /tmp/artifacts.

Remove the chgrp portion of the chown calls as it shouldn't be necessary
and it keeps the user running sti from being able to delete the temp
dir.

Change /tmp/artifacts from 755 to 775 so it's group writable, meaning it
should be able to be deleted by the host user.

Fixes #164