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.
This seems very fragile relative to packer updates. Can we just make this verify that it starts with `usage: packer` and maybe the exit status?