Making cereal play nicely when saving enable_shared_from_this

cereal no longer permanently modifies the state of internal workings of
std::enable_shared_from_this when saving.  cereal *should* be completely
compatible with both saving and loading anything that inherits from this now.
This fixes issue #68 - note that there is still a minor issue regarding
classes declared final that will run into a bug with the way we check for
enable_shared_from_this (see issue #65).

Issue #65 will be addressed in the future by changing the way we check
for derivation from enable_shared_from_this.  In the current scheme, we
can detect this even if you use protected inheritance.  In the future, cereal
will not be able to get around protected inheritance of enable_shared_from without
befriending cereal::access.  This will come at the benefit of allowing classes
declared final to be used with polymorphic serialization.

unittests.cpp

@@ -1262,6 +1262,8 @@ void test_memory_load_construct()
       oar( o_shared3 );
     }
 
+    o_shared3->shared_from_this(); // tests github issue #68
+
     decltype(o_shared1) i_shared1;
     decltype(o_shared2) i_shared2;
     decltype(o_unique1) i_unique1;