Update the Domain struct with the current fields

I had to temporary disable the Renew because it was relying on a field that is no longer part of the domain model.
Temporary workaround for Renew

RenewWhoisPrivacy is not part of a Domain struct, but for now let's keep it here, since it's used for the Renew method.
Sort the Record fields according to the official serializer
Early return if the request creation failed

Do not attempt to set any header.
Document NewRequest
Use a namespace-based approach for API calls

I like the approach adopted by go-github where you use

    dnsimple.Domans.Create

instead of

    dnsimple.CreateDomain