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/speed_test_nested_array.cpp

@@ -73,7 +73,7 @@ void test_array_of_array() {
     std::cout << "Start converting..." << std::endl;
     {
         boost::timer::cpu_timer timer;
-        unpacked.get().convert(&v2);
+        unpacked.get().convert(v2);
         std::string result = timer.format();
         std::cout << result << std::endl;
     }