Updated to remove most of the warnings in Visual Studio. (There's still the one about all control paths returning a value left.) Fixed one warning (when an istream converts to void * to then convert to bool), and disabled three.

include/node.h

@@ -20,7 +20,7 @@ namespace YAML
 	class Content;
 	class Scanner;
 	class Emitter;
-	class ParserState;
+	struct ParserState;
 
 	enum CONTENT_TYPE { CT_NONE, CT_SCALAR, CT_SEQUENCE, CT_MAP };