acme/challenge_request handles only 1 domain name

No other functions inside acme/ handle lists, so
to keep the class consistent, challenge_request()
should only handle a string parameter. Iteration
should be moved to the client, which is handling 
the list of domain names anyway.
According to spec, `name` MUST be a domain name. This only verifies it's not an empty string, but I think the validation belongs here. Would something like `if not is_hostname_sane(name):` be better?