Refactor #from_xml_node

I remember I wrote most of the comments back when to make the code clearer to future me and all of us using the project. I now want to simply make the code better by making defining methods for each of the various process operations:

* process_node_as_supported_type
* process_node_as_xml_content
* process_node_with_custom_parser
* process_node_with_default_parser

This should be viewed after Pull Request #31 . This work has some small dependency on that work.