2.5.1 codegen issues

[Benson Margulies]

So I apparently busted some things in the codegen plugin for 2.5.1.
Unfortunately, I've been sick for a week, and that really puts a chomp
into my ability to roll my sleeves up. My strong belief is that we
need proper plugin integration tests, the same way that other maven
plugins deal with this -- the maven-invoker-plugin. Has anyone else
got time to start on this? If not, I'll get onto it when I can.