Unicode characters in file names result in incorrect navigation links

[bryanbraun]

Issue originally reported by @morberg.

I pushed two new directories, one with and one without accented characters, to my starter book and the one with a file name containing 'ä' ends up with an incorrect link in the navigation (links to http://morberg.github.io/Äccented/äccented.html instead of http://morberg.github.io/starter-book/Äccented/äccented.html). Manually entering the correct link works fine.

[morberg]

Looking at the generated HTML the link with an accented character begins with '/' (which it shouldn't). The generated link is "/Äccented/äccented.html", but should be "Äccented/äccented.html".