Tweak compiler classpath and generated jar contents

[jodersky]

• Add full sbt support
• Fix class-path inclusion issue in compiler (and tweak error message)
• Generate jar with all produced sources: in Scala, a single source file may generate several classfiles if it contains non-equivalent java constructs, such as case classes, objects, traits etc
• Boyscout:
  • clean unused dependencies in build
  • remove unused code from NotebookExporterSpec