Addng relative path variable for layouts to use to be able to refer to shared resources


 - Added sv_rel_base variable that expands to relative path to
   base of generated website for layouts to use. Added docs and
   some tests.

 - Tweaked some of the perldoc to remove some warnings (replaced =method with =item)

 - Changed mkpath in tests to File::Path::make_path in the faint
   hope that is causing the tets failure on Windows