Add paybox system integration

Hello,
We are using active merchant for the paypox_system french payment method.

This payment method are different and change the classic usage of active_merchant helper...

PayboxSystem are very strict with parameters, so we have created a method that return a hash with only the needed params. (`query_with_hmac`)

We have documented the usage of the paybox helper so it can be used without `payment_service_for` helper.

Thanks