TLS fixes & detailed install instructions

I fixed up the remaining certificate-related issues from #34 and #26. It now correctly verifies the cert when connecting with `openssl s_client -connect mail.lukecyca.com:993` (and also port 465), and also shows `Trusted TLS connection established` in `mail.log` when connecting out to other mail servers.

@bcachet, can you test these out and confirm that they work for you as well?

I wanted to write up some more explicit instructions on what exactly goes in each cert file, and ended up adding similar detail for the other manual setup steps. @al3x, feel free to copy edit. :)