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

test/object.cpp

@@ -72,7 +72,7 @@ TEST(object, convert)
     msgpack::unpack(ret, sbuf.data(), sbuf.size());
 
     myclass m2;
-    ret.get().convert(&m2);
+    ret.get().convert(m2);
 
     EXPECT_EQ(m1, m2);
 }