enable roundcube's password-change plugin

add warnings about using a fresh machine because I wont support installing this on an existing machine

List a DKIM and SPF testing tool in the documentation.

Seems like it was removed in nsd4 (since Ubuntu 14.04). It now
silently does nothing.

Fixes #41.

http://pingability.com/zoneinfo.jsp for checking DNS and MX servers.

Added DKIM and SPF testing tool to 'Checking that it worked' section.

move the ssh PasswordAuthentication check to after installing ssh

instead of installing roundcube from Ubuntu (before replacing it), just install the dependencies we need

I had previously awkwardly disabled it because on my VPS the necessary kernel
module wasn't available. I meant to make it an option, but instead I took
out the apt-get install. Now it works for me so I'm putting everything back.

Force the scripts to run in bash

nsd doesn't install without creating the `nsd` user first - merge branch from randallsquared-master, fixes #25

fixes #23

Fix install of php5-cgi, roundcube SQL directory on Ubuntu 14.04

Not sure if I've tested this. Also because of graylisting there's not much of a need
to do sophisticated spam filtering, at least based on the email I get.

DNS test: also check a public nameserver to verify that the registrar (or up-host) configuration is correct too