Added method to request invoice as XML

By adding this method, it is possible to easily retrieve data that is otherwise unavailable through the API, such as the KID number of an invoice.
`xml` seems like such a generic name. The actual SOAP action is named `get_oio_xml`, we aim to follow E-conomics naming as much as possible, so `get_oio_xml` or at least `oio_xml` would be more apt here.