Introduced visitors into datamodel and JSON converter

* New interfaces ValueVisitor and SnakVisitor can be used to avoid
casting objects of these types
* Renamed ConverterImpl to JsonConverter; restricted public interface to
methods for handling EntityDocuments; removed Converter interface as it
is not needed
* Split JsonConverter into several classes by extracting the respective
visitors
* Extracted constants from JsonConverter
* Renamed "jsonconverter" package to "json"
* Updated tests accordingly
Fixed typo in JavaDoc
Move EntityDocumentProcessor to datamodel package

This interface is not specific to dumpfiles. For example, it might be
useful for implementing a class that writes serialized JSON to some
stream.
Fixed module name in license header

Seems that this does not get updated by the Maven plugin.
Merge remote-tracking branch 'origin/master' into datamodel-visitors

Conflicts:
	wdtk-datamodel/src/main/java/org/wikidata/wdtk/datamodel/implementation/SomeValueSnakImpl.java
Merge pull request #55 from Wikidata/datamodel-visitors

 	Introduced visitors into datamodel and JSON converter