Fixing support of final classes in polymorphic pointers

This fixes issue #65, however, this comes at a mild consequence for
anyone wishing to serialize types that inherit from std::enable_shared_from_this.
cereal no longer supports this inheritance if done using the protected access
modifier without befriending cereal::access.

For example, consider all of the following (pretend they are polymorphic)
serialized with some smart pointer:

struct A : std::enable_shared_from_this<A> {}; // OK
struct B final : protected A { friend class cereal::access; }; // OK
struct B2 final : protected A { }; // ERROR: cereal can't access enable_shared_from_this
struct C : A {}; // OK
struct D {}; // OK - a normal class with no enable_shared_from_this
Merge branch 'develop' of github.com:USCiLab/cereal into develop