Added unattended install of postfix. Fixes #37

Could potentially add two things here:

* template for /etc/aliases, where user can define an off-server email to which any system emails should be sent
* template for /etc/postfix/main.cf, with optional support to relay to external smtp server (eg: gmail)
Added mail.postmaster var and /etc/aliases template

See http://www.feep.net/sendmail/tutorial/intro/aliases.html
Merge pull request #72 from genesis/37-postfix-mta

Added unattended install of postfix