Nearly done with DifferenceEngine.java; probably only one more evening's work to go on it.

At the rate I'm getting through this stuff, I'm hoping to have a fully compilable (albeit probably nonfunctional) version in maybe ten to twelve weeks. I am limited as to the amount of work I can do on weekdays. There are only so many hours in the day, after all. Once it all compiles, then I get to the real fun: making it work at all.