Add creation of test source/javadoc JARs to Maven build

So next time they will be part of the deployed release. It does not hurt
not to have them in 1.1, but now we have a complete template for how to
do a proper Maven Central release with
  - application binaries + JavaDocs + sources JARs
  - test binaries + JavaDocs + sources JARs
All in all 6 JARs.