Addresses #3
Added support by adding parallel code wherever `$PUBLIC_IP` was used.
Providing an IPv6 address is completely optional.
Playing around on my IPv6-enabled mail server revealed that — before
this change — mailinabox might try to use an IPv6 address as the value
for `$PUBLIC_IP`, which wouldn't work out well.
* use a public box (the official Ubuntu 14.04 box which contra the description does have VBox Guest Additions installed)
* now that we allow SSH password logins, since Vagrant requires it, dont muck with sshd_config here
* don't put the machine on the public network because that will allow anyone to log into it with Vagrant's default username/password, duh
* adding a Vagrantfile
* in a non-interactive setup like this, create the user's first email account for them
* let the machine auto-detect its IP address using http://icanhazip.com/
* use our own justtesting.email domain to provision a subdomain for users so they can quickly get started
