Add initializer by URL in README
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

content_owner.claims also accept `where` to only list claims that
match a set of IDs, such as:

    content_owner.claims.where id: 'A1234,B1345'
    content_owner.claims.where asset_id: 'A123'
    content_owner.claims.where video_id: 'x1234'
    content_owner.claims.where q: 'something in the title'