Added unattended install of postfix

Fixes #37, verified it can send messages and implements sendmail command.

We could potentially add extra functionality in the following ways:

* A template for [`/etc/aliases`](http://www.feep.net/sendmail/tutorial/intro/aliases.html) and a var `mail.postmaster`, to define an off-server email address to which any system emails should be sent (rather than going unread and filling up the server spool)
* ~~A template for `/etc/postfix/main.cf` and smtp credentials under a `mail` dict to relay all email to an external smtp server (eg: gmail); or alternately install something like nullmailer (instead of postfix) that *only* relays mail~~ Writing this off as feature creep