Added giropay.de payment plugin

Added giropay.de (german payment processor) payment plugin for the joomla-payments API. 

It's PHP CodeSniffer valid (Joomla code style) and seems to work fine so far (it has been actively used in Matukio from compojoom.com since 2 weeks). It requires curl (The included giropay SDK does).

Kind regards,

Yves
compojoom.com