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.
We already have a checkTxOutputs() function, but that checks both the address and amount of every output. Can you see if it'd be possible to combine them in a single one and use it everywhere to avoid duplication? If that's not possible, I think this one should be renamed to checkTxOutputAmounts() and then the other one can call it, avoiding some of the duplication