Osx support for adding users/groups

##### Issue Type:

Feature Pull Request

##### Ansible Version:

```
ansible 1.8 (devel b7082677de) last updated 2014/08/29 00:02:43 (GMT +200)
```

##### Environment:

All, target Darwin Mac OS X

##### Summary:

Add basic users and groups management for Darwin Mac OS X hosts.

##### How to use:


Same as for other hosts.

Please note that this version does not yet support encrypted user
passwords. Be aware of security concerns.