[COOK-1679] Create a flag to append users to group

http://tickets.opscode.com/browse/COOK-1679

Instead of simply replacing all the members of the group, this allows
for appending users to the group.

Example run:

```
[Wed, 19 Sep 2012 13:26:11 +0000] INFO: Storing updated cookbooks/users/resources/manage.rb in the cache.
[Wed, 19 Sep 2012 13:26:11 +0000] INFO: Storing updated cookbooks/users/providers/manage.rb in the cache.
[Wed, 19 Sep 2012 13:26:12 +0000] INFO: Storing updated cookbooks/users/recipes/sysadmins.rb in the cache.
[Wed, 19 Sep 2012 13:26:12 +0000] INFO: Storing updated cookbooks/users/metadata.rb in the cache.
[Wed, 19 Sep 2012 13:26:12 +0000] INFO: Storing updated cookbooks/users/CONTRIBUTING in the cache.
[Wed, 19 Sep 2012 13:26:13 +0000] INFO: Storing updated cookbooks/users/LICENSE in the cache.
[Wed, 19 Sep 2012 13:26:13 +0000] INFO: Storing updated cookbooks/users/README.md in the cache.
[Wed, 19 Sep 2012 13:26:13 +0000] INFO: Storing updated cookbooks/users/metadata.json in the cache.
[Wed, 19 Sep 2012 13:26:14 +0000] INFO: Storing updated cookbooks/users/CHANGELOG.md in the cache.
[Wed, 19 Sep 2012 13:26:55 +0000] INFO: Processing users_manage[admin] action create (users-ext::admins line 2)
[Wed, 19 Sep 2012 13:26:56 +0000] INFO: Processing user[gchristensen] action create (/srv/chef/file_store/cookbooks/users/providers/manage.rb line 75)
[Wed, 19 Sep 2012 13:26:56 +0000] INFO: user[gchristensen] created
[Wed, 19 Sep 2012 13:26:56 +0000] INFO: Processing directory[/home/gchristensen/.ssh] action create (/srv/chef/file_store/cookbooks/users/providers/manage.rb line 92)
[Wed, 19 Sep 2012 13:26:56 +0000] INFO: directory[/home/gchristensen/.ssh] created directory /home/gchristensen/.ssh
[Wed, 19 Sep 2012 13:26:56 +0000] INFO: directory[/home/gchristensen/.ssh] owner changed to 2001
[Wed, 19 Sep 2012 13:26:56 +0000] INFO: directory[/home/gchristensen/.ssh] group changed to 2001
[Wed, 19 Sep 2012 13:26:56 +0000] INFO: directory[/home/gchristensen/.ssh] mode changed to 700
[Wed, 19 Sep 2012 13:26:56 +0000] INFO: Processing template[/home/gchristensen/.ssh/authorized_keys] action create (/srv/chef/file_store/cookbooks/users/providers/manage.rb line 99)
[Wed, 19 Sep 2012 13:26:56 +0000] INFO: template[/home/gchristensen/.ssh/authorized_keys] owner changed to 2001
[Wed, 19 Sep 2012 13:26:56 +0000] INFO: template[/home/gchristensen/.ssh/authorized_keys] group changed to 2001
[Wed, 19 Sep 2012 13:26:56 +0000] INFO: template[/home/gchristensen/.ssh/authorized_keys] updated content
[Wed, 19 Sep 2012 13:26:56 +0000] INFO: Processing group[admin] action modify (/srv/chef/file_store/cookbooks/users/providers/manage.rb line 112)
[Wed, 19 Sep 2012 13:26:56 +0000] INFO: group[admin] modified
```