Adding the initial plugin files and folders.
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`.
Hide the "color" label with JS and remove an unneeded <br />.
Make sure the plugins classes can't be extended for the first version.
Inline doc updates.
Make the 'wp-color-picker' script a dependency for our custom backgrounds script.
Better methods for validating and sanitizing the background post data.  Also, a couple of bugs cleaned up.
Earlier check for whether the post type supports 'custom-background' so that we're not loading JS unnecessarily.
Prefix the custom field keys with `_`.
readme.md overhaul.
Version bump to 0.1.0-beta-1.
Rename the plugin to "Custom Background Extended".
Rename everything to match the new plugin name.
Update the readme.md file to reflect plugin name changes.
Version bump to 0.1.0-beta-2.
Add the selected background to the pool of uploaded backgrounds for the currently active theme.
Use the correct attachment ID and conditional check.
Allow the user to click the image in the meta box to open the frame for selecting a new image.