handle PUT/POST/DELETE to /data/fs/[path] (Fixes #140)

Replacing the earlier PR (#152)
The local `rename` and `delete` functions here could be promoted to the top-level as `move` and `delete`, but accept paths instead of collections. Since a collection can always be converted to a path (though the reverse is not true), you can just promote the local collection names to paths and invoke the top-level move / delete functions.