Added serializing and deserializing for monolingual text values.
Also: setup work.
Tested MonolingualTextValue
Added support for Labels and Descriptions
Tested aliases.
Streamlined tests.
Added Serializing/Deserializing of ItemIds.
Added SiteLinks
Fixed Typo in class name of "TestItemDocument"
Moved structuram comparision of Json strings in seperate class.
Cherry-pick from create-parser-of-constraints
Fix parsing of parameters with nowiki tag
Fix template scanner to properly parse nowiki tags
Merge branch 'master' of https://github.com/Wikidata/Wikidata-Toolkit into serialization-of-constraints
Change in package structure.
Speedtest.
Fix scanning of templates with nowiki tags
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
further work on the external JSON serialization.
Added Snaks, tests for novalue- and somevalue snaks
Finished datavalues
Finished first draft of the (de)serialization.
Organized classes in a nicer package structure.
Reduced code replication.
Created a test suite for convenience.
Introduced property (de)serialization