Use temporary files during download

This prevents unfinished files from lying around when downloads are
interrupted.
Small fix in RDF serialization example
Simpler ways for iterating over WDTK data

* Classes that are mainly collections of other objects implement
Iterable (e.g., Reference).
* Classes that have nested collections of objects provide direct
iterator access to all elements (e.g., to iterate over all statements in
an item document).
* A helper class NestedIterator is used to iterate over nested
collections.
Add dependency to wdtk-util to use NestedIterator
Update RELEASE-NOTES.md
Merge pull request #86 from Wikidata/partial-dump-downloads

Partial dump downloads
patch some javadoc warnings
Merge pull request #92 from Wikidata/javadoc-fixes

patch some javadoc warnings
Merge branch 'master' into iterable-data

Conflicts:
	RELEASE-NOTES.md
Fix buffer size computation
Merge pull request #88 from Wikidata/iterable-data

Simpler ways for iterating over WDTK data
Merge branch 'master' of https://github.com/Wikidata/Wikidata-Toolkit into serialization-of-constraints