Some cleanup

* Remove Sonatype parent pom; it causes warnings in Eclipse and does not
really make the release process much simpler
* Rename and merge datastructure packages in wdtk-storage (no point
having two packages here)
* Removed unused benchmarks package in wdtk-examples
Changed imports to match renaming in previous commit
Merge branch 'master' of https://github.com/Wikidata/Wikidata-Toolkit into create-classes-to-measure-the-memory-usage