New version of nsdiff

A few people have shown interest in nsdiff which is more than I expected for a quick hack. In fact, Terry Burton at the University of Leicester is planning to put it into production! He also reported a bug (it got TTL changes wrong) and said they needed TSIG support so they can control which view a zone is transferred from. So I have fixed those problems and even added documentation! Maybe I should give it a home page...

(Previously, previously.)