Testing showed that it may take a few seconds for the default values to
populate.  If the help text is shown, “Enter the public IP address…,”
but no prompt is shown, the user may get confused and try to enter the
IP address before mailinabox has had a chance to figure out and display
a suitable default value.

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)