Skeleton implementation

no documentation at all.
`warnings.warn` is *not* for this kind of things.
> warnings.warn() in library code if the issue is avoidable and the client application should be modified to eliminate the warning
> logging.warning() if there is nothing the client application can do about the situation, but the event should still be noted
> https://docs.python.org/2/howto/logging.html

In this case, the developer can do nothing to prevent it. It's up to the user to choose sane paths.

So logging.warning is a better choice (but please don't log under the root logger; use `logging.getLogger('fsdb').warning` instead