Added support for setting default desktop picture

A users desktop picture can now be set as part of the kitchen plan provision. 

The image can exist on either the local file system or be loaded from a remote url.

```
recipes:
    mac_os_x:
        - osxdefaults::set_desktop_wallpaper
attributes:
    osxdefaults:
        desktop_picture: http://placehold.it/1920x1080&text=kitchen+plan
```
The file will be placed `~/Library/Desktop Pictures/kitchenplan.png`

I also added a helper to allow recipes to have access to the current os version.