Remove XmlDecoder/XmlEncoder

I'm in the process of checking how we can make tomboy-library becoming a portable class library (PCL). The XmlTextReader/XmlTextWriter classes are right now not supported. While digging into that, I found the XmlEncode/XmlDecode classes seem not to do anything or serve any purpose except providing some Xml options. I've refactored the settings part into a customer XmlSettings class and removed the XmlEncode/XmlDecode classes. 

As I am not sure if I am missing something here (maybe they did serve a purpose?) I wanted a second opinion. If there are no objections, feel free to merge.