Rake task to add and remove child groups to groups

http://projects.puppetlabs.com/issues/12969

New tasks:
```shell
$ rake nodegroup:addgroup name=parentgr group=childgr
Group 'childgr' added to node group 'parentgr'
$ rake nodegroup:listgroups name=parentgr
childgr
$ rake nodegroup:delgroup name=parentgr group=childgr
Group 'childgr' removed from node group 'parentgr'
```

Aiming for master branch, we'll merge through to rails3.