Pass form data on to omnipay gateway

Let the gateway implementation decide what's relevant, rather than pre-filtering on gateway agnostic layers like PurchaseService
Merge pull request #33 from silverstripe-iterators/pulls/form-data-gateway

Pass form data on to omnipay gateway
Coding conventions: One class per file

No functional changes
Explicit marking as offsite gateway through isOffsite()

Some offsite gateways don't separate between authorize and complete requests,
so we need a different way to determine they're off site in the first place
without kicking off a purchase request within omnipay.
Docs
Set omnipay response earlier in PurchaseService

It might be used in onCaptured callbacks
Merge pull request #40 from silverstripe-iterators/pulls/omnipay-response-earlier

Set omnipay response earlier in PurchaseService
Allow overriding of credit card implementation
Setting omnipay response earlier in PurchaseService
Adding initial refund() method on RefundService
Merge pull request #44 from halkyon/refundservice

Adding initial refund() method on RefundService
Merge pull request #43 from silverstripe-iterators/pulls/response-earlier

Setting omnipay response earlier in PurchaseService->purchase()
Added info about how to set test mode
Merge pull request #41 from silverstripe-iterators/pulls/creditcard

Allow overriding of credit card implementation
Merge pull request #36 from silverstripe-iterators/pulls/class-per-file

Coding conventions: One class per file
Merge pull request #38 from silverstripe-iterators/pulls/isoffsite

Explicit marking as offsite gateway through isOffsite()
Fixed issue with payment development admin
fixes #45
Added default menu priority to PaymentAdmin
API: Added getPayment() function to GatewayResponse
Minor tidy