Refactored Node.get_absolute_url and related functions (such as MultiView.reverse...

Refactored Node.get_absolute_url and related functions (such as MultiView.reverse) to use a new Node.construct_url function, which handles constructing any type of url involving a node or raises an appropriate error if this is not possible. Moved MultiView.reverse to View and merged View.get_subpath into it. Added APPEND_SLASH support to node_view, including support for resolving non-philo targets (resolves issue 75). Made urlpatterns for penfield and waldo MultiViews unambiguous. Altered LazyNode to always return a subpath of at least "/". Added handles_subpath methods to View and MultiView.