Remove unused DocDataPaymentMethodSerializer.
Remove old commentted out code in orders.js.
Cleanup Order Payment flow doc.
Add supports_single as config parameter for DocData payment methods.

Also add direct debit config to default settings.
Cleanup variable name in DocData payment adapter.
Add API for DocData DirectDebit.
Merge pull request #311 from benkonrath/direct-debit-api

Direct debit api