Fixed #399

If MSGPACK_DISABLE_LEGACY_CONVERT is defined, msgpack::object::convert(T*) is removed.
Added MSGPACK_DISABLE_LEGACY_CONVERT to build system and documents.

Please define MSGPACK_DISABLE_LEGACY_CONVERT and update your code as follows:
Replace
  int i;
  obj.convert(&i); // Removed pointer version
with
  int i;
  obj.convert(i);  // Reference version

example/cpp03/simple.cpp

@@ -40,7 +40,7 @@ int main(void)
     // convert msgpack::object instance into the original type.
     // if the type is mismatched, it throws msgpack::type_error exception.
     msgpack::type::tuple<int, bool, std::string> dst;
-    deserialized.convert(&dst);
+    deserialized.convert(dst);
 
     return 0;
 }