Added user_launchers resouce

Based on *user_apps_autostarts* with slight modifications. 
Tested successfully with the following chef-json input:

```json
{
    "run_list": "recipe[gecos_ws_mgmt::default]",
    "gecos_ws_mgmt": {
        "users_mgmt": {
            "user_launchers_res": {
                "users":[
                    {
                        "username":"vagrant",
                        "launchers": ["firefox.desktop", "gedit.desktop"]
                    }
                ]
            }
        }
    }
}
```