client.CertificateManager: handle too long subject line for overly-long hostnames

Example might be:
  https://2459d6dc103cb5933875-c0245c5c937c5dedcca3f1764ecc9b2f.ssl.cf2.rackcdn.com/

Hack is to generate wildcard certificate, so long last name doesn't matter.
I guess this should work for nearly all long names, as they usually have
similar wildcard-CN themselves anyway.
README: note on the last wildcard-CN fix