Add content_owner.claims to list Content ID claims

A new Claim class is introduced to list YouTube ContentID Claims.
See https://developers.google.com/youtube/partner/docs/v1/claims
@stavro I've restored the option to channel `content_owner.claims.where(id: '123')` and added a test.

I just feel weird that the parameter is called `id` but the doc says 

> The id parameter specifies a list of comma-separated YouTube claim IDs to retrieve.

It should be called `ids`! :sleepy: