Add compiletime attribute (+Add more ChefSpec tests) (rebased)

Issue #13 rebased after #14:

> I added a `node['freebsd']['compiletime_portsnap']` attribute [like in the apt cookbook](https://github.com/opscode-cookbooks/apt#attributes). I've also fixed some issues related with some tests.

All the changes included in this PR:

* Add `compiletime_portsnap` attribute: 2a58a3a
* Add more ChefSpec tests: 589e891
* Update ChefSpec to 4.1: de24aef
* Add Gemfiles for Serverspec: 527f49d

Some notes:

* The ChefSpec tests rebase has been a bit painful. I tried to keep your test descriptions over mine.
* I leave `node['freebsd']['compiletime_pkgng']` out of this PR to avoid more complexity. It will probably be a future PR.