client.RenewCertificate now supports CSRs, and in fact prefers them,
when renewing certificates. In other words, if the certificate was
created via a CSR then using that will be attempted before re-generating
off a new private key.
Also adjusted the API of ObtainCertificateForCSR to be a little
more in line with the original ObtainCertificate function.
Add 2 new functions to acme.Client for registration stuff:
* QueryRegistration: This performs a POST on the client
registration's URI and gets the updated registration info.
* DeleteRegistration: This deletes the registration as currently
configured in the client.
The latter, while a part of the IETF draft, may not be 100%
functional in LE yet, my tests showed that resources were still
available after deletion.
* new ChallengeProvider with Present and CleanUp methods
* new Challenge type describing `http-01`, `tls-sni-01`, `dns-01`
* new client.SetChallengeProvider to support custom implementations
