Added support for returning owner's group name in stat module

##### Issue Type:

Feature Pull Request

##### Ansible Version:

1.7.1

##### Environment:

N/A

##### Summary:

Added support for returning owner's group name in stat module.

This would enable Ansible users to use stat module to get the group name of a file or a directory. (The current stat module is only able to get the gid.)

###### Usage:

    - name: "Get a file's group name"
      stat: path="/somepath/somefile"
      register: st
    
    - debug: var=st.stat.gr_name