Merge pull request #81 from sfe-ev/sonderwurst

Wunderschön! Handle German Sonderwurst in SEPA XML files.
Looking at the doc, we will need to have the same "trick" applied for other countries. It seems they are as many standards as they are countries ;)
Cleanup: drop unused `payment_instrument_id` field

Creditor doesn't have this field anymore in the New World Order.
Cleanup: drop unused variable
Don't rely on $form->_values in postProcess hook

This is only available when editing an existing Payment Processor
record, but not when creating a new one. Thus, we need to use other ways
to get at the values:

 * 'user_name' is available from $form->_submitValues

 * The Payment Processor Type can be fetched by the ID in $form->_ppType
   using the API. (We could also get the ID from _submitValues -- but as
   we get the convenient _ppType too, let's use it :-) )

 * The 'payment_processor_id' (i.e. the ID of the newly created PP
   record) is not available anywhere in the postProcess hook. Instead,
   we have to fetch the record from the DB (by the uniqe `name`) using
   the API.
Only try retrieving existing creditor when editing existing PP record

Don't run the API 'get' with an unset $ppid, as this gets all existing
creditor records, rather than getting none. (Resulting in bogus values
being pre-filled when adding a new record.)