use **kwargs in ItemPage.addClaim to support edit summary

The Wikibase extension (used on Wikidata) has recently
enabled the edit summary on entities. ItemPage.editEntity and
Claim.addSource already support it, but ItemPage.addClaim doesn't.

Change-Id: Id70a0be601f603238c8eed817cc5fc58c4705c88
Merge "use **kwargs in ItemPage.addClaim to support edit summary"