Added support to create scheduled downtimes with past dates

* pandora_console/godmode/setup/setup_general.php,
  pandora_console/include/functions_config.php: Added a config token
	to allow the creation of scheduled downtimes in the past.

* pandora_console/godmode/agentes/planned_downtime.editor.php: Now
	is possible to create a scheduled downtime with a past date
	if the config token is activated.