DefaultRapidXMLReaderNode

@simon.ottenhaus @mirkowaechter I've added the DefaultRapidXMLReaderNode. I've chose a design where such an object which represents multiple nodes which override each others values (the first node is the default node) can only be created from a RapidXmlReaderNode. This prevents unnecessary duplication of code and hopefully makes it easier to use, since you only have to create one RapidXmlReader for all purposes.

Do you think that is not intuitive enough? And I am not sure how to preferably describe the functionality of the method RapidXmlReaderNode::get_node_with_default(const std::vector<char*>& node_names). Maybe you have some ideas.