Initalize payment extension

Create all files needed by a payment extension for Isotope. The payment
can be used at webshop act as cash payment.
Skeleton work as payment service without hidden trigger.

The skeleton redirect now to an payment service stub for input credit
card information and redirect back to shop. Hidden trigger
(postsale.php) is not concerned with it.
Merge pull request #1 from falkoschumann/develop

Develop