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'd originally thought of extending currentTx to keep a map of outputs to the list of inputs added to fulfil them. That would make it simpler to rollback an output, and testing would probably be easier as we wouldn't need to mock calcFee. Maybe you considered something like that but found other reasons to prefer this approach instead?