Added <base> tag to make relative links work

This is the fix that works for me, and in some way I think this is correct.  
- Every link which is generated by the templates can put in the correct link itself
- All links generated by markdown or Rst etc. have a stable point to reference.

But if you find another fix or a better one, by all means use that.
This is just to show how it can work.

Wim Oudshoorn.