Custom generator cookbook names

:construction: I want to clean up the error messaging when `ChefRunner` fails before I consider this done.

* Cleanup a broken test
* Allow generator cookbooks to have arbitrary names
* make the argument to `-g` the path to the cookbook rather than the path to the directory containing the cookbook. The old way works for compatibility; we're still pre-1.0 so we could break it if we want, but this seems to be a somewhat popular feature and backcompat isn't hard in this case.
Is this intentional?