small improvement
XmlNode:addChild return reference to added node
xmlfiles sample
Json parser + StringReader

- Json parser can be based on MOAIJsonParser or on lua custom parser.
Results are practically the dames but lua parser allows to be easily
extended
- Basic StringReader class (to be extended)
- Utils/Externals -> Json and XmlParser are moved here
- Better ldlc comments (IO, Ini, xml, json)