* add issuer certificate to CertificateResource
Also write it out to the file system when running "lego run"
Removed caching of the issuer certificate inside the acme client, since
it didn't appear to be used.
* only append issuerCert to issuedCert in case of success
Effectively a no-op since issuerCert will be nil on error, but it seems
more correct to only do it if fetching the issuer succeeds.
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
