use path.posix when dealing with manta paths

this fixes a bug on windows when a dir is lazily created using `client.put()`

ie. `client.put('/foo/bar/baz')`: If this yields a `DirectoryDoesNotExistError`, it will attempt to:

``` js
client.mkdirp(path.dirname(path.normalize('/foo/bar/baz')))
```

(as expected).

However, on Windows, this will result in `client.mkdirp('\\foo\\bar')`, which will fail when the request is made to manta.  This issue can be avoided by using `path.posix` for any and all path related calls that deal with manta paths.

discovered by @papertigers - https://github.com/bahamas10/node-manta-sync/pull/14