Challenge names now have their own type and constants

proxy: add a insecure_skip_verify option - closes #320

letsencrypt: Fix perm of user key

gzip: Add .svg to default ext list

Implements "on-demand TLS" as I call it, which means obtaining TLS certificates on-the-fly during TLS handshakes if a certificate for the requested hostname is not already available. Only the first request for a new hostname will experience higher latency; subsequent requests will get the new certificates right out of memory.

Code still needs lots of cleanup but the feature is basically working.

I've built this on Go 1.6 beta 1 and made some changes to be more compatible. Namely, I removed the use of the /x/net/http2 package and let net/http enable h2 by default; updated the way h2 is disabled (if the user requires it); moved TLS_FALLBACK_SCSV to the front of the cipher suites list (all values not accepted by http2 must go after those allowed by it); removed the NextProto default of http/1.1; set the http.Server.TLSConfig value to the TLS config used by the listener (we left it nil before, but this prevents automatic enabling of h2).

It is very likely there is more to do, but at least already Caddy uses HTTP/2 when built with Go 1.6.

Also only set custom address if alternate port is specified (rather than using a blank address; just cleaner this way)

gzip: Fix for wrong content-type when templates is used.

This change fixes the scenario where you reload the config and it tries to obtain a cert from the ACME server, but no email address is found or terms have not been agreed to in-process. This is unfortunate but it should not stop the server from reloading, so we assume empty email address in this case.

This prevents serving HTTPS over port 80 or HTTP over 443. It's confusing and we don't allow it.

It is unexpected to serve localhost on port 443 or any server on 443 if TLS is disabled, even if the port is blank. Also don't warn about how to force TLS on the HTTP port.

The docs link to this structure and all its methods related to the browse template; keeping them together makes it possible to link to the whole block of code that is relevant.

Doesn't test the SkipInsecureVerify proxy setting, but that can be done at another time.