Omnipay eWay integration

[agileware]

eWay payment processors are already included in the extension, but currently not fully implemented.

PR coming through with a working implementation. This was written to "just work", so suggestions on improvements are more than welcome.

[eileenmcnaughton]

I feel like eway is pretty functional in the extension now - although I've mostly been testing the Rapid processor - which processes payments via a form that posts offsite