Added authentication, TLS support and keyring support for SMTP

Also updated README (might conflict if previous pull request that renames README to .md extension has been merged). If so, fix is trivial or reject and I will rebase against upstream/master (thp:master).

Yes - that is embarrassing. I don't have a non-authed place to test. Fixed in moons/urlwatch@9743561700f538efab50d96ba0ca8616d71afe72 . I'm new to pull requests - what's the best way to proceed?