#5 Make state load more robust

[felixwatts]

At the moment the state is loaded using a XmlReader which means that it is very intolerant of missing or unexpected nodes in the XML. As a result, special handing is required for previous version of the state file format and this is prone to errors. Would be better to use a XmlDocument (or similar) and XPath and fill in missing data with default values, which should allow complete backwards and forwards compatibility of state file.

Note: this is already done for config so reuse where possible.