Deprecate use of boost::shared_ptr in favour of std::shared_ptr

[stefanseefeld]

Assuming boost::shared_ptr and std::shared_ptr are distinct types, let Boost.Python provide builtin support for std::shared_ptr whenever the compiler supports it, and optionally support for boost::shared_ptr (for backward compatibility). The latter may be removed later, to reduce Boost.Python's dependency on Boost.

[eudoxos]

This depends on #29, i.e. making std::shared_ptr support complete (in particular, implicit upcasing, which works for boost::shared_ptr but not std::shared_ptr).

[stefanseefeld]

Support for std::shared_ptr has been added as per 97e4b34.
This still needs documentation.