Support for creating and realizing virtual users and groups.  Requires '(#15081) Make virtual resources work with create_resources' which will only be available in Puppet 3.1 so will only merge into master then.
Merged in default branch changes
This is a copy of the Puppet 3.1.0 create_resources function that finally supports virtual resources.  Added here so that this module can be used by older Puppet version.
Use a boolean to determine whether or not to create virtual users or groups.  The default is now to create virtual resources.
Provide a way for lists of virtual users and groups to be realized.
Documentation, examples and changelog update for switch to virtual users and groups by default.
Merge branch 'virtual'

By default virtual user and group resources are now created that must then
be realized rather than real resources as before.  This allows all of the user
and group configuration to be added to a common part of the hiera hierarchy
but only the relevent users and groups created on each host.