Improve dump downloading behaviour

* Fix issue #63 by checking availability explicitly
* Finding most recent dump of some type now checks availability
* Better logging output to show which dumps are found/processed
* Updated tests to work with new code
Document bugfix in release notes
Merge pull request #65 from Wikidata/issue-#63

Improve dump downloading behaviour
Merge branch 'master' of ssh://github.com/Wikidata/Wikidata-Toolkit into create-classes-to-measure-the-memory-usage