s/posixGroup/groupOfNames/g
removed useless gidnumber uniqueness + added memberof overlay
Taking into account PR's remarks

users have to be created before groups so that the overlay takes the
membership into account.
Taking into account PR 3

setting the reference integrity check flag to false.
Adding a fake user on each groupOfNames objects
Merge pull request #3 from georchestra/groupofnames

s/posixGroup/groupOfNames/g