Create Omnipay connector

[judgej]

Omipay has a SagePay V2 driver, but not V3. It would be good (once this one is completely tidied up) to create a connector to expose it as an Omnipay driver. The driver would support all the base functionality that Omnipay provides, and then could provide extensions to the additional functionality that Omnipay does not generally support (or encourage), such as additional basket information, additional customer details, separate emails for billing and shipping.

The transport would need to be abstracted properly (it is currently stuck in ServiceAbstreact::postSagePay as a curl-based method). This would allow us to use the Omnipay transport mechanisms.

The network flow diagram here would help to understand how this would all link together:

https://github.com/judgej/omnipay-sagepay/tree/patch-1/docs

See:

https://github.com/thephpleague/omnipay