Add a tfXMLDocument::Write(const char*) function

Writes the xml dom to a given file
Add Set methods for node/attribute name/value
Add {Append,Insert,Remove}{Child,Attribute} functions
Modify tfXMLNode Set and Get values

PugiXML uses special 'xml_text' nodes to handle the text value inside
nodes. This modification abstracts away from that model, and the setter
and getter methods for xml nodes work as expected.