Wraps apt::unattended-upgrades

Closes #12.

I had to wrap the recipes:
  * the default uses mailutils instead of bsd-mailx
  * the 50unattended-upgrades template did not set reboot_time

Opened MR's for both:
 * https://github.com/opscode-cookbooks/apt/pull/108
 * https://github.com/opscode-cookbooks/apt/pull/107

You'll also notice I've redefined resource package[mailutils] as there's
a discrepancy between git repo and what supermarket api serves us. I
opened an issue for that here:
 * https://github.com/opscode-cookbooks/apt/issues/109