Use path/filepath instead of path (for Windows)

The code was using path.Split, path.Join etc. instead of filepath.Split, filepath.Join. path assumes '/' separators, but Windows of course uses '\'. Talk about the gift that keeps on giving...I've been fixing bugs like this in portable code since 1983.

See docker/machine#76

Signed-off-by: John Gossman <johngos@microsoft.com>