Add getClaim method to Statement

This is preparatory work for changing the relation between Claim and
Statement from inheritance to composition, as listed on
https://github.com/wmde/WikibaseDataModel/issues/22
Because then it's still an instance of `Statement`, and some code right now might be doing `if ( $claim instanceof Statement )`. You are right that `return $this` will suffice for a lot of cases. In any case, this is a temporary implementation.