Handle XML declaration

[skreutzer]

Handle the XML declaration properly and provide it's data to the caller (via the still missing StartDocument class). Use the XMLEventReader::HandleProcessingInstruction() and XMLEventReader::HandleAttributes() methods for it, even if people claim that the XML declaration isn't a processing instruction, but still it's basically a processing instruction for the XML processor/reader/parser (maybe with the only limitation that it needs to be the very first "node" in the data stream).