Put ScctPlugin into a namespace

[copumpkin]

It's remarkably hard to get at top-level types if you're living inside a custom package. Our build process is fairly involved and our Build.scala has a package declaration in it. This makes it really painful to get at ScctPlugin, and caused me much lost time when trying to diagnose the issue.

The resolution would just be to move ScctPlugin into reaktor.something, like the scct compiler plugin. Then we can import that from Build.scala and all will be well. I'm not sure how build.sbt will deal with that, of course...

[kscaldef]

This would really be nice, and seems in line with most other sbt plugins. As it stands, this is a near deal-breaker for us as far as using SCCT. (I'm still trying to work out how to use reflection to hack around this)