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.

Protect private key from being world-readable

Postfix, Dovecot, and nginx all read the key file while they're running
as root — before dropping permissions — so no authorization is needed on
the private key file beyond being root-readable.

Merges branch 'mkropat-populate-hostname-ip'

add comments to the new get_default_hostname etc. functions, and simplify the logic in the Vagrantfile and start.sh so that we always call into the same two functions

Default IP+hostname values were incorrect for my VPS provider. I
improved the detection, which should give correct results results for
almost any provider. Specific issues addressed:

- icanhazip.com detection was only enabled in non-interactive mode
- `hostname` is by convention a short (non-fqdn) name in Ubuntu
- `hostname --fqdn` fails if provider does not pouplate `hosts` file
- `hostname -i` fails if provider does not populate `hosts` file
- `curl` without `--fail` will someday return crazy results
  when icanhazip.com returns 500 errors or similar

Mask password input on stdin in tools/mail.py

allow dashes in emails during validation, and for aliases allow a much wider range of characters, fixes #64

* for local mail users, also disallows periods at the beginning or end of the local or domain parts
* Dovecot gets confused if the string contains any unusual characters, so local mail users are restricted to a narrow regex
* for mail aliases Postfix is not confused so use a regex based on RFC 2822

* 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

move the SSH password login check out of setup because it interfers with Vagrant and into a separate script that we'll use for auditing in a later phase

* 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

script to check the SSL certificate, with instructions for turning the self-signed certificate into a properly signed certificate

* Created a new Python/flask-based management daemon.
* Moved the mail user management core code from tools/mail.py to the new daemon.
* tools/mail.py is a wrapper around the daemon and can be run as a non-root user.
* Adding a new initscript for the management daemon.
* Moving dns_update.sh to the management daemon, called via curl'ing the daemon's API.

This also now runs the DNS update after mail users and aliases are added/removed,
which sets up new domains' DNS as needed.

remove permit_dnswl_client because postfix has odd behavior when an IP address is not listed: it turns all bounces into deferrals (retry)

partially reverts 6d473f81

test_mail: if EHLO test fails continue testing the rest, since user may be waiting on DNS propagation

add more postfix rules: reject_non_fqdn_sender, reject_unknown_sender_domain, reject_rhsbl_sender, and permit_dnswl_client

test_dns: properly test the non-primary domain of a box (for email addresses on domains besides PUBLIC_HOSTNAME)