Add Claims#list and ClaimSearch#list

Adds the functionality to list Claims, and search for Claims.

https://developers.google.com/youtube/partner/docs/v1/claims/list

https://developers.google.com/youtube/partner/docs/v1/claimSearch/list

The way we handle attribute assignment is slightly different between each model class.  Some assign to instance variables, some don't.  I just kept this in line with some other models, but I could see some room for improvement on the claim attributes.

Thoughts?