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.
Support correct site links

* Extend dump file management to support gzipped dump files that do not
contain page revisions
* Simplify some methods in the file/Web utils for better support of
various compressions
* Simplify and generalize some methods in the dump file management;
fewer method names refer to specific dump files now
* Support new dump type "SITES" for sites table dump
* Add Sites interface and implementation to datamodel to hold and use
sites data
* Add new dump file processor for parsing SITES dumps to get Sites
object
* Add example program that shows how instrument all this to make use of
sites data
* Renamed MwDumpFileProcessorImpl to MwRevisionDumpFileProcessor, since
we have several such classes for several types of dumps now
* Updated processing code and tests to work with the changes
Extended Javadoc
Improved Javadoc
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
Sorry doing a fault with merging, correct it here
Fix VerifyError, issue #58
Merge pull request #60 from Wikidata/datamodel-visitors

Add toString() methods lost in previous merge
Merge pull request #59 from Wikidata/fix-issue-#58

Fix VerifyError, issue #58
Merge branch 'master' into sitelink-support
Allow larger time differences in timer test

This should largely avoid random test failures even under very strange
conditions. If not, we can always increase the new tolerance constant
even further.
add qualifier-order, check for missing keys in JsonResultComparer
just formatting
Call finishProcessingEntityDocuments() when finishing

This fixes a bug that prevented the respective EntityDocumentProcessor
method to be called, e.g., in DumpProcessingExample.
Merge pull request #62 from Wikidata/finish-entity-processor-fix

Call finishProcessingEntityDocuments() when finishing
Merge pull request #57 from Wikidata/sitelink-support

Sitelink support (merge only after #55)