add: top navigation bar: ability to use raw urls

instead of a view name, use something like /foo
permits to add a Nagvis link for instance
bugfix: /contrib needs a RequestContext
add: nagvis example in settings.py
fix: active links in topmenu bar, with direct urls
Merge pull request #375 from matthieucan/bugfix-topmenu-conf

add: top navigation bar: ability to use raw urls