Fix singleton implementation bugs
New includes/admin folder

Move assets, images and admin files
Refactor use of Settings API

No changes in settings fields. @todo Clean up use of register_setting()
Introduce version, activation and updater funcs

Move standalone admin functions to admin/functions.php and move output
RSS method to includes/functions.php
Introduce options.php and remove uninstall.php

Move uninstall functions back inline and more admin functions to
admin/functions.php.