Get the core Credit Memo mappings based on docs at https://developer.intuit.com/docs/95_deprecated/qbd_v2/qbd_v2_reference/0600_object_reference/creditmemo

Signed-off-by: Michelle Santarsiero <michelle.santarsiero@gmail.com>
Ensure that a line item is initialized

Signed-off-by: Michelle Santarsiero <michelle.santarsiero@gmail.com>
Associate Detail with Windows module PaymentDetail

Signed-off-by: Michelle Santarsiero <michelle.santarsiero@gmail.com>
Add in credit memo service and spec

Signed-off-by: Michelle Santarsiero <michelle.santarsiero@gmail.com>
Removing OperationNode from CreditMemo; using helper copied over from other repository in our specs