Reorder error enum

so that it is possible to check for no error or need more data by doing <=
NeedMoreData
Add at_beginning flag to QObjectNode::insertNode

Not rally the way to go, but usefull since JT::ObjectNode has some implicit
order by using std::vector
Make the Tokenizer::releaseCallback callback be called

when there is no buffer registered. Then the argument is null and can be used
to do the initial read