Factor out a common trait for stage2 plugins

[megri]

May do with a better name than stage2 too

[cvogt]

Let's not rename for now. stage2 symbolizes that this is the second stage of cbt's scala code during bootstrapping. On the package level, everything ends up in cbt. Happy to have a discussion about renaming suggestions though. So far I had imagined introducing one more stage for technical reasons, renaming nailgun_launcher to stageSomething and then renumbering them all 1-4 or 0-3. Maybe...

+100 for refactoring the compiler plugins. Maybe by moving the code that add compiler plugins into a ScalaBuild plugin trait ( into we can move some of the things in BaseBuild eventually as well ). And then having that trait maintain a list of plugins, all of which it simply enables, and you can add more. It probably needs to be a list of artifactId, groupId, version, resolverUrl, name. We can put the known ones into a list in the companion: ScalaBuild.plugins.
That's one way to do it. Any alternatives or thoughts?