fix backslash path when running on Windows

One of my co-worker meeted an error while he worked on the Windows machine.

	Possibly unhandled Error: PUT https://myobjectstorage.domain.com/v1/somepath\myContainerName responded with statusCode: 400, body:

The main reason is that `path.join()` used in the `slash()` converts `/`s to `\`s when it runs on Windows. I think that the node.js `path` is designed for local path.