Serve /usr/share/javascript at /_sysassets/javascript/ in notebook

This makes it possible to use a local installed mathjax at
/usr/share/javascript, which is the default location on Fedora.

See http://fedoraproject.org/wiki/Packaging:JavaScript, especially the
Server Location section.

I found at least a discussion about the `/_sysassets/javascript` location also on debian:
http://comments.gmane.org/gmane.linux.debian.devel.javascript/3849

Don't know, if there is some kind of default installation path on windows though...