add support for deploying chef to FreeBSD

Adding support for FreeBSD was fairly simple. To determine if the user was on FreeBSD or Linux, I use `uname -o`.

There are a couple of caveats:

* FreeBSD requires us to use /bin/sh as our shell
* FreeBSD's root user's group is `wheel` not `root`, but on OSX, Linux and FreeBSD systems, the GID is 0, so use the GID instead of the group name

There are a couple of notes that need to be made for FreeBSD hosts (namely the user you ssh in as needs to use a `sh`-style shell, not `tcsh`, and you need to install sudo), but I'm not sure if you'd want that added to the README or the wiki.

Thanks for the work on littlechef.