Merge branch 'dw/cast-graph'

Merge branch 'dw/cast-graph'

* dw/cast-graph:
  Suppress unused variable warning.
  Use false : x : 0 idiom to avoid evaluating x.
  Hold a weak pointer in instance_holder when ownership is released.
  Test that adopt() leaves a weak pointer in the Lua object.
  Remove use of conversion_storage.
  Handle NULL smart pointers.
  Test object identity with shared_ptr_converter.
  Make shared_ptr_converter public.
  Fix bug in id allocation.
  Rework local_id allocation in class_id_map.
  Handle null pointer in shared_ptr_converter.
  Use add_cast() for wrappers as well.
  Add missing <luabind/wrapper_base.hpp> include.
  Add tests for new inheritance code.
  Add make_instance() helper function.
  New inheritance graph code.
  Add missing detail/ref.hpp include.