IDEA source dirs for generated sources (#251)

Instead of subdirs of all `managedSourceDirectories`, use subdirs of `sourceManaged` as well as `managedSourceDirectories` themselves

As I said in conclusion of the discussion at #252, I'd like to submit a different fix for #251.

This time around, source roots include:
 - the subdirectories of `sourceManaged` (in the considered config), which I think is more in line with the sbt doc at http://www.scala-sbt.org/release/docs/Howto/generatefiles.html than adding all the subdirs of all `managedSourceDirectories`, event though the latter setting is initially a list containing only `sourceManaged`.
 - as well as the directories in the `managedSourceDirectories` list. Even though the doc for that setting could be seen as ambiguous ("Managed source directories, which contain sources generated by the build."), I still think it reasonable to expect that a directory added to `managedSourceDirectories` (either by a plugin or directly in a build) be a compile root. Going through a handful of codegen plugins, I saw only one (sbt-protobuf) that adds anything to `managedSourceDirectories`, and it is indeed a compile root that is added.

I've tested this fix with the project that originally prompted me to raise the issue (which uses sbt-protobuf and sbt-buildinfo), as well as with the sample project given with issue #209, and I find it to be working as expected in both cases.