Allow capturing a postfix "rest of path" as route parameter

Allows the following syntax for route strings:
   "/users/:userid/files/::path"

A ::-prefixed name may be supplied last in the string, and in the
example above this allows matching on strings like this:
   "/users/123/files/Pictures/Summer2012"
where userid="123" and path="Pictures/Summer1012"
or
   "/users/123/files/"
where userid="123" and path="".
(so unlike other parameters, this one can be empty)