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.
Yeah, I also couldn't think of names for them that would make the
relationship immediately obvious. After experimenting a bit, though, I'm
starting to wonder if maybe it wouldn't be better to have two separate
tests instead of a single one. There'd be some duplication, but we could
refactor a thing or two and keep that to a minimum, and I think it'd
make the test a lot easier to follow. Something like this:
https://gist.github.com/gsalgado/e92627abd2533df5b38c

What do you think?