* refactor: create log.Infof and log.Warnf
* refactor: review DNS providers.
- use one `http.Client` by provider instead of one client by request
- use the same receiver name `d` for all `DNSProvider`
- use `http.MethodXXX`
* refactor: logger init.
If TCP connection reuse (KeepAlives) are left on then, in a sequence
of challenges arising from a multiple-domain certficate, challenges
after the 1st can fail due to reusing the now defunct tcp connection
used in the first challenge. This has been observed when using the Go
standard library reverse proxy to forward the challenges to Lego.
Fixes#107
* 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
