After 10 certificates are issued, no new certificate requests are allowed for 10 minutes after a successful issuance.

Biggest change is no longer using standard library's tls.Config.getCertificate function to get a certificate during TLS handshake. Implemented our own cache which can be changed dynamically at runtime, even during TLS handshakes. As such, restarts are no longer required after certificate renewals or OCSP updates.

We also allow loading multiple certificates and keys per host, even by specifying a directory (tls got a new 'load' command for that).

Renamed the letsencrypt package to https in a gradual effort to become more generic; and https is more fitting for what the package does now.

There are still some known bugs, e.g. reloading where a new certificate is required but port 80 isn't currently listening, will cause the challenge to fail. There's still plenty of cleanup to do and tests to write. It is especially confusing right now how we enable "on-demand" TLS during setup and keep track of that. But this change should basically work so far.

proxy: Support unix sockets

fastcgi: New function DialWithDialer to create FCGIClient with custom Dialer.

Making directives externally registerable

templates: Add .Markdown directive

This allows any template to use:
{{.Markdown "filename"}} which will convert the markdown contents
of filename to HTML and then include the HTML in the template.

markdown: enable definition lists

When the requested host is not found, log the remote host.

letsencrypt: properly retrieve hostname from request.

fastcgi: IPv6 when parsing r.RemoteAddr

basicauth: fixed 'go vet' printing function value

More tests are needed for the other environmental variables.
These tests were specifically made for testing of IP addresses.

fastcgi: Parse address from fastcgi directive, pass results to Dial()

This allows scheme prefixes "tcp://" and "fastcgi://" in configuration.

Fixes #540

wrap lines to 80

also update copyright year.

Challenge names now have their own type and constants

proxy: add a insecure_skip_verify option - closes #320