Remove References interface

Removes the References interface and all its usages by replacing them
with ReferenceList. This can also be easily done in Wikibase.git and
all other packages depending on the DataModel so it should not be a
blocker for the update to 2.0.
Fix StatementTest (silly netbeans)
Merge pull request #233 from wmde/newstatementconstructor

Use new version of Statement constructor
Merge pull request #232 from wmde/removereferencesinterface

Remove References interface
Remove old Statement constructor