Add JS-Powered Upgrader

Make sure we provide an option to automatically upgrade seoslides-imported background images.  This mechanism will use AJAX to step through each presentation and pull up any attached background images (only imported BG images will have the presentation as a parent) and flag them as imported.
Cover a pre-release version too
Merge in latest dev updates

Conflicts:
	css/front-end.web.fade.css
	css/front-end.web.horizontal.css
	css/front-end.web.none.css
	css/front-end.web.vertical.css
	js/seoslides_list.min.js
Allow partial updates to automatically resume
Prompt when leaving the settings page if an upgrade is in progress
untypo
Merge branch 'upgrade' into dev