Change config FTP default to FALSE. Set backup params in cron.php. Add CRON constant to wrapping 'if' statement

First off, I continue to find Fuel CMS excellent and hope my small contributions help a little! ->
If ftp fails then script stops and emails etc are not sent. Perhaps better to have ftp default as FALSE, since the other ftp params are empty meaning the ftp will fail. 
Set backup params in cron.php to respect settings/config. Without this patch, only the class config settings are used.
Add CRON constant (defined by the cronjobs module ci_crons.php) to wrapping if statement for compatibility.