Refactoring: reorg Maven modules for reusability

[andrus]

Let's reorganize our module structure to simplify JJava kernel reusability. The new structure might look like this:

• jjava-distro
  1. Implements main
  2. Builds the fat jar out of jjava-jupyter, jjava-kernel, jjava-maven
  3. Shades dependencies
  4. Packages the fat jar and jjava-launcher
• jjava-kernel - most of the Java code from ex. jjava. Classpath magics. Pure library, clean dependencies. No shading or fat jar.
• jjava-jupyter - a renamed jupyter-jvm-basekernel
• jjava-maven - Maven magics
• jjava-launcher - unchanged