Standard pages and enhancements for payment modules

Added the required stuff to provide a set of standard pages for payment modules:
- payment accepted (the good old merci.html)
- payment rejected for some reason (the good old regret.html)
- a "go to gateway page", to submit the form to the payment gateway.

An AbstractPaymentModule class has been added, to provide modules convenient methods to get success and failure payment URLs, and generate the form sent to the gateway.

A BasePaymentModuleController class has been added, to process gateway callback request, and, if the module require it, redirections to standard pages. It provides convenient methods for changing order status (not paid -> paid on payment confirmation, and paid -> not paid on a further payment cancelation), redirecting to success or failure pages, and writing to a specific log file.