* Initial implementation of ZeroSSL API issuer
Still needs CA support for CommonName-less certs
* Accommodate ZeroSSL CSR requirements; fix DNS prop check
* Fix README example
* Fix comment
Eliminates a bajillion nil checks and footguns
(except in tests, which bypass exported APIs, but that is expected)
Most recent #207
Logging can still be disabled via zap.NewNop(), if necessary.
(But disabling logging in CertMagic is a really bad idea.)
This is necessary to eliminate confusing naming conventions, since now
we have Manager types, having an issuer called ACMEManager was
confusing.
CertificateManager is a redundant name as this package is called
CertMagic, so that a Manager manages certificates should be obvious.
It's also more succinct. Plus, it's consistent with Issuer which is not
named CertificateIssuer.