Ticket cron

Added new cronjob to find Prices via API call

* Default configuration updated to supply price URL and API targets
* Added cronjob `ticketupdate.php` that does the actual update
* Modified `setting.class.php` to allow writing a new setting or updating existing ones
* Added new `tools.class.php`, used only in crons so far, for generic cron functions in crons
* Updated header template to display new value
* Updated some configuration variables in `config.inc.dist.php` to allow customizable chaininfo and blockexplorer URL used in Templates