correct group verifier to check a group exists on the system

You cannot verify a group exists with `id -g` alone.
It's commonly incorrectly documented that you can run `id -g groupname` infact it is for displaying user and group information of the calling process, ie the current user, or if passed a specific user.
`usage: id -AGnMPgrpu [user]`

This patch correctly implements checking whether a group exists on a unix system.

Fixes #179 

Note:
I decided against using `getent` because it is no longer available on OSX since Apple started using DirectoryService instead of NetInfo