Add copyright notice to std_shared_ptr_converter.hpp.
shared_ptr_converter: Use existing value converter.
Add intrusive_ptr_converter.

intrusive_ptr_converter can be used for all smart pointer types where
construction of multiple smart pointers from a single raw pointer is not a
problem, such as boost::intrusive_ptr (for which specializations of
luabind::default_converter are provided).

Includes test.