Add some serverspec tests

This should make it easier to confirm whether PR's work.  Just run `kitchen test` and it'll verify packer is installed correctly.  

4 simple tests:
* Looks for the packer executable
* Makes sure packer executable is returnable in the PATH
* Try a packer command and check version (currently a regex against any 0. release)
* Try a packer command without arguments and check default output.

Hope this helps, let me know if you have any issues with it.