Support for std::enable_shared_from_this

Fixes #47

When we detect a shared_ptr being loaded in load_and_allocate that
also is of a type that derives from enable_shared_from_this, extra
work is done to save the state of the enable_shared_from_this before
the user gets to meddle with it via placement new inside of
cereal::allocate.  State is restored after getting back from the user.