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.
Removed unnecessary whitespace
Don’t use explicit file encoding hint
Don’t write unit tests for non-optimal code paths
Merge pull request #92 from willoller/master

acme/challenge_request handles only 1 domain name