Fix erroneous build error when non native directory separator is used.

The build would fail if
* a page contains an internal hyperlink
* and, the target page path, in the pages config, uses a different
  directory separator than the one native to the platform
Fix conversion from path to url for Windows.

On Windows a path such as about/license.md would yield the the url
about/license\ this had an effect on the creation of relative paths
. Resources such as css and js, as well as internal hyperlinks,
would get incorrect urls.