Add rollback last output functionality

Note that this does not include the transaction finalization which is
part of the rollback last output procedure in the opentransactions
wiki.
I think this should be named something like createTxWithFakeInputs() as I believe it's more important to highlight the fact that we use fake inputs than that we return a currentTx (now decoratedTx, really).

Also, it's not obvious to me why you need a tx with TstFakeCredits? Would it be possible to have it use actual Credits (which can be created with TstCreateCredits) instead?