Change 'cb_background_image' custom field to 'cb_background_image_id'.
Always set background image options if the user selects a background.  Don't allow this to default to the WP custom background settings because this won't be consistent from theme to theme or even consistent between changes to the WP custom background.
Overhaul of much of the code.  Added custom `wp_head` callback and an option for `position_x`.