Allow a quantity in the BUY JSON

[charafantah]

Allow a QTY field in the buy JSON in order to be able to buy multiple items from the same product.

From the ethernet terminal, this value will always be 1. But from the new WiFi terminal you can buy multiple items.

[charafantah]

Instead of allowing quantity. we should create another REST handle to allow a different type of BUY.
/BUYQTY for example that can handle multiple buy requests. This would allow it to remain backward compatible.