refactor WbUrl and UrlRewriter to drop requirement for having a WbUrl start with /

Changes WbUrl forms:
/2013/im_/example.com -> 2013/im_/example.com
/*/example.com -> */example.com
/example.com -> example.com

* also simplify scheme-agnostic url (//) handling by just eating up extra
slashes
* add additional doctests on route, with and w/o custom SCRIPT_NAME