removed obsolete (fbml-specific) code from fb_canvas.module.  Improved documentation and eliminated php notices.

Change the fb_cb url rewrite syntax.  Now use apikey instead of label.  This helps simplify fb_settings.inc.
multiple simplifications (fixes, lets hope) to fb_settings.inc.
fb_session.inc no longer needed (at least for time being).
more sanity checks and output in fb_devel.module