XmlSettings class (similar to JsonSettings)

A patch hopefully to give the xml side of things some love.
I've copied JsonSetting and JsonResponse as much as possible for consistency. 

The XmlSettings is to allow setting default options when serializing xml. In my case, setting the Encoding.

I thought that could potentially break some people's usages, so I've added a boolean property EncodingEnabled and defaulted it to false. Other than that DefaultEncoding is set to utf-8 just like JsonSettings.

Hope it's all ok and confirms to everything...