Reviewed and documented Jackson Value implementations
Reviewed and documented Jackson SiteLink implementation
Reviewed, documented, and refactored

Main changes:
* Distinguish "read only" helper classes from those used by Jackson
* Simplify several type conversion methods
* Removed helper classes that did not do anything special and used
standard datamodel implementations instead
* A lot of additional documentation
* Removed Helper; put its methods closer to where they are used
* Removed some constant definitions that occurred in two classes with
the same values
Never catch exceptions in tests

JUnit is well prepared to deal with exceptions and report errors in a
useful way. No need for code to do anything else there.
Reviewed and documented
Prepare injection of site IRI into termed documents

The termed document's method setSiteIri() can be used after JSON
deserialization.
Relate Jackson statements to parent document; update tests

* Jackson statements are related to documents rather than to subject
entities now, to support site IRI injection later
* Fixed several issues with the test construction (naming conventions,
code structure)
Use empty item document as parent document of statements in testing
Support site IRI injection into JSON dump processing
Introduce factory and copy constructor helper for Jackson objects
Improved tests and fixed many smaller issues to make them run

* Order of reference snaks and qualifiers now used
* Some ordering happening in toString() to ensure uniform output
* Jackson-based Snaks and ItemIdValues have parent documents to find the
site IRI
* The Jackson factory correctly sets all data (including mock parent
documents where needed)
* Some missing toString/equals/hashCode added
Change order of equality assertions to exercise methods of tested code
Remark on new JSON format support
Merge pull request #91 from Wikidata/externalJsonReader

Full support for reading API-based JSON format
Merge branch 'master' into cleaner-examples

Conflicts:
	wdtk-datamodel/src/main/java/org/wikidata/wdtk/datamodel/json/jackson/serializers/AliasesDeserializer.java