translate Paths to valid dot-separated MongoDB collection names (Fixes #139)

That would make things easier for the API, but it is working as implemented. `ls` returns *all* files and dirs, and the API is filtering that list to just the stuff that is contained in the requested path. It's certainly cumbersome, and other backends will likely provide an actual way to enumerate a single "directory," so I think it makes sense to move that logic into the filesystem.