various bugfixes

* new file:   favicon.ico
	add

* modified:   lib/feeds/spip.php
	fix plugin for when the short desc is empty (accept content
	with empty text)
	change autoupdate interval to 1 hour

* modified:   lib/lib.php
	in dom_get_text, check if the passed $dom is an object.

* modified:   lib/signlib.php
	remove debug message in session

* modified:   screen/screen-zone.php
	check if the returned content is empty, fail gracefully

* modified:   sql/signage.sql
	fix get_next_feed function to take target into account