Remove usage of deprecated Entity methods

Two issues remain in this code:

* Code only works with items and properties (this is now explicit,
while before it would just break for entities with statements). This
is the issue described in https://lists.wikimedia.org/pipermail/wikidata-tech/2014-June/000489.html
And can be fixed as follows https://lists.wikimedia.org/pipermail/wikidata-tech/2014-August/000546.html

* The deprecated Claims class is still used. This forces transformation
from StatementList to Claims to StatementList, which is a waste of resources.

Change-Id: I37843beca7de1a629c3da392644a8abe21866473
Merge "Remove usage of deprecated Entity methods"