Add a MessagePayload trait, and implement it for Body and ChatState.

src/chatstates.rs

@@ -1,6 +1,7 @@
 use minidom::Element;
 
 use error::Error;
+use super::MessagePayload;
 
 use ns::CHATSTATES_NS;
 
@@ -13,6 +14,8 @@ pub enum ChatState {
     Paused,
 }
 
+impl MessagePayload for ChatState {}
+
 pub fn parse_chatstate(root: &Element) -> Result<ChatState, Error> {
     for _ in root.children() {
         return Err(Error::ParseError("Unknown child in chatstate element."));