Add paybox system integration

[ntalbott]

Issue by spk Monday Sep 23, 2013 at 10:08 GMT Originally opened as https://github.com/Shopify/active_merchant/pull/851

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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

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spk included the following code: https://github.com/Shopify/active_merchant/pull/851/commits