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.