Added group_create function to postgres module

Allows for creating a postgres "group". Groups and Users are both Roles
in postgres. Groups simply cannot login while Users can.
Added option to user and group create in postgres module

Can now provide a string of groups (comma seperated) to place the new
user/group in.
Added function to update a group in postgres

Added ability to provide groups for a user update and function to update
a group
Added function to remove a group
Merge pull request #3287 from shadowfax-chc/postgres-group

Added some support for postgres groups to the postgres module