Added smart handling of HEAD request.
Exported some internal features of WSF_REQUEST and WSF_RESPONSE to respectively WSF_REQUEST_EXPORTER and WSF_RESPONSE_EXPORTER
The documentation is built only on demand.
A mapping entry can be hidden for the documentation
One can change the status code when building itself the WSF_ROUTER_SELF_DOCUMENTATION_MESSAGE
Added WSF_ROUTER_MAPPING.associated_resource
Added WSF_ROUTER_SELF_DOCUMENTATION_HANDLER and WSF_ROUTER_SELF_DOCUMENTATION_MESSAGE to provide a self documentation for WSF_ROUTER (for now, only HTML)