Various bugfixes and improvements

Hi there, Cameron. I have made a bunch of improvements to the module. It's hard to summarize it all. There were a few bugs that I've fixed (including #40865 on CPAN's bug tracker) that prevented the module from producing valid output, at least as far as my bank was concerned. I've also tried to improve the documentation, since the actual standard is not readily available, and to make the internals a bit simpler and more consistent.

Are you able to make a new release at CPAN? If not, I maintain a couple of other banking-related CPAN modules and would be happy to do the release if you would add me (sseverin) as a co-maintainer at pause.perl.org.

Regards,
-Steve