Reliably calculate one path level per description level

Okay, I _think_ I might have achieved what we both wanted from #16, in a consistent and reliable way implemented simply. 

There will always be one path component for each level of description: `MinitestVcr::Spec` as a description remains `MinitestVcr::Spec` and turns into `MinitestVcr_Spec` in the file system, instead of two levels `MinitestVcr/Spec`.  While still behaving consistently everywhere else. 

Figured out we could get what we needed out of looking at the `desc` for each description level one by one, instead of trying to parse the `class.name` which has extra stuff in it and has lost the information needed. 

What do you think?