Configured packaging properly

* Cleaned up assembly specifications for wdtk-distribution, using
dependencies instead of static, hard-to-maintain file paths
* Configure aggregate javadoc generation and use this in the
distribution (instead of copying unpacked files from many sources on top
of each other, overwriting existing files)

Preparing packages works with "mvn clean install -Psign" as before.
Merge branch 'master' of https://github.com/Wikidata/Wikidata-Toolkit.git