BNL Positivity backend

This adds support for the BNL POSitivity (BNP Paribas) e-POSitivity virtual POS payment gateway.
It is a major italian payment gateway, official documentation (in italian language) can be found at the following addresses:
http://www.bnlpositivity.it/it/soluzioni-pagamento/pos-virtuale-connect.html
http://www.bnlpositivity.it/shared/allegati-pubblici/Manuale_Connect_2.6_per_il_sito.pdf