should use `path.sep` instead of `/` for cross-OS compat

[blahah]

Right now paths are created using / to join and split paths - this makes the library not compatible with node fs on Windows. If all the / path construction and parsing was changed to use path.sep, it would be OS tolerant I think.

[martinheidegger]

It might be a good idea to turn / into a constant (for code-reading-sake) but the purpose of the append-tree API is that system-independent logs are written and this separator really is a "append-tree" separator; not-os-specific. Any API that uses append-tree to create folders should be doing this.