In-a-dyn v2.3

Changes

• Distribute CONTRIBUTING.md in release tarballs, by André Colomb
• Clean up debug messages for HTTPS connections, by André Colomb
• New build-depends, libgnutls28-dev for Debian/Ubuntu users and GnuTLS >= 3.0 for others, by André Colomb
• Issue #192: Add examples/*.conf to source distribution, by André Colomb

Fixes

• TCP, not UDP, for getaddrinfo() hints + numeric lookups, by André Colomb
• Disable SSL for checkip connections to SPDYN service, by André Colomb
• Issue #186: Allow IPv6 for HTTP(S) connections, by André Colomb
• Issue #189: Ignore premature session termination in GnuTLS, by André Colomb
• Issue #193: Fix broken internal links in README.md, by André Colomb