Improved compatibility of mkdirpSync
Merge branch 'pr/173'

* pr/173:
  Improved compatibility of mkdirpSync
Fix spacing.
Make mkdirSync more robust.

This version does not do any path splitting or concatenation on its own, but instead simply uses path.dirname() to get parent paths until it can no longer go up any higher. This will happen either at '/' (on *nix) or e.g. 'C:\' (on Windows) for absolute paths, and at '.' for relative paths.