Started overriding toString methods on the document level.
This is a first draft for String representation of WDTK-datamodel
objects.
Merge pull request #56 from Wikidata/add_toString_for_DataModelImpl

Add to string for data model impl
Merge remote-tracking branch 'origin/master' into datamodel-visitors

Conflicts:
	wdtk-datamodel/src/main/java/org/wikidata/wdtk/datamodel/implementation/SomeValueSnakImpl.java