improved handling of shared/smart pointers
Previously, pybind11 required classes using std::shared_ptr<> to derive from std::enable_shared_from_this<> (or compilation failures would ensue). Everything now also works for classes that don't do this, assuming that some basic rules are followed (e.g. never passing "raw" pointers of instances manged by shared pointers). The safer std::enable_shared_from_this<> approach continues to be supported.