Add compiletime attribute (+Add more ChefSpec tests)

I added a `node['freebsd']['compiletime']` 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` attribute: c6b4c9e
* Fixed FreeBSD version in the ChefSpec tests: 354850f
* Added more ChefSpec tests: 1ab0bb2
* Added Berkshelf and ChefSpec to the Gemfile: d997f76
* Update to Serverspec 2: b1e7a07