Re-designed user types dispatching mechanism.

msgpakc-c 0.5.9 or older uses ADL.
msgpack-c 1.0.x uses overloading with header files ordering.
msgpack-c 1.1.x uses functor with class template specialization.

test/msgpack_cpp11.cpp

@@ -1,4 +1,4 @@
-#include <msgpack_fwd.hpp>
+#include <msgpack.hpp>
 
 #include <gtest/gtest.h>
 
@@ -29,8 +29,6 @@ public:
 
 MSGPACK_ADD_ENUM(TestEnumClassMemberClass::TestEnumClassType);
 
-#include <msgpack.hpp>
-
 using namespace std;
 
 const unsigned int kLoop = 10000;