Fix retrieval of `creditor_id` in batchContribution()

Don't try to find the `creditor_id` from the `payment_instrument_id`,
which is no longer related in the New World Order.

Conveniently, the `creditor_id` is actually available directly from the
mandate object, which we get as a parameter... No need for any extra
queries.

This also drops the gross hack of creating a fake creditor on the fly.
No idea what that was for, but it doesn't apply anymore, and I *very*
much hope it's not needed. (Works fine here without.)
Use correct field name when fetching creditor ID from TxGroup object

For extra fun, the field is called `sdd_creditor_id` here, rather than
just `creditor_id` like everywhere else. Using the right name gives us
the actual ID, which in turn makes the creditor lookup actually work.

(Previously, it just retrieved the single creditor when there was only
one, and went BOOM otherwise.)
Merge pull request #80 from sfe-ev/multi-creditor

Fixes for handling multiple creditors