Some improvements and update to 1.9.2

+ Add new function for reveal hidden tinymce buttons (styleselect) in
new file custom_shortcode.php
+ Add button for insert <!--nextpage--> quicktag in the editor
+ Retrieve number $posts_per_page from wp backend configuration for
blog.php template
+ Add rel canonical if SEO Yoast and AIOSP are not installed
(cleanup.php)
+ Add rel next and prev for paginations (cleanup.php)