Merge pull request #1134 from opscode/CHEF-3297

CHEF-3297: Add excluded_members property to the group resources
Conflicts:

	chef/lib/chef/provider/group.rb
	chef/lib/chef/provider/group/usermod.rb
	chef/lib/chef/resource/group.rb
	chef/spec/unit/provider/group/pw_spec.rb
	chef/spec/unit/provider/group/usermod_spec.rb
	spec/functional/resource/group_spec.rb
	spec/spec_helper.rb
	spec/support/platform_helpers.rb