Xysti::user_check() and Xysti::page_meta()

Implemented Xysti::user_check() and Xysti::page_meta(). Page title is no
longer required in meta. It will default to the ucwords of the segment.
Xysti::sitemap_page() renamed to Xysti::sitemap_page_walk() to improve
legibility, and order of arguments switched.