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 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