Introduced SnakGroups

The datamodel was updatd so that all lists of Snaks are grouped
by the snak property. This affects statement qualifiers and references.
Due to the latter, this also fixes issue #44.
Documented changes
Fixed bug in JSON conversion of snak lists

EntityType was used instead of EntityId for grouping snaks in JSON.
Interestingly, the test cases were specifically checking for the wrong
output as well.
Merge pull request #53 from Wikidata/datamodel-updates

Introduce SnakGroups into data model
Merge remote-tracking branch 'origin/master' into fix-issue-#53

Conflicts:
	wdtk-dumpfiles/src/test/java/org/wikidata/wdtk/dumpfiles/JsonConverterTest.java

To accomodate for the data model changes.
Added the missing JsonConverterTest again.
No idea why this got lost.
Somehow, the changes in JsonConverterTest got lost, (also in master) so
here are the changes made during the introduction of snak groups.
Fix unit test
Merge pull request #54 from Wikidata/fix-issue-#53

Fix issue #53
Merge remote-tracking branch 'origin/master' into
rewrite_JsonConverterTest

Conflicts:
	wdtk-dumpfiles/src/test/java/org/wikidata/wdtk/dumpfiles/JsonConverterTest.java