presence: Make PresencePayload a trait, and implement it on every payload.

src/muc/muc.rs

@@ -5,6 +5,7 @@
 // License, v. 2.0. If a copy of the MPL was not distributed with this
 // file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
+use presence::PresencePayload;
 use date::DateTime;
 
 generate_element!(
@@ -36,6 +37,8 @@ generate_element!(
     ]
 );
 
+impl PresencePayload for Muc {}
+
 #[cfg(test)]
 mod tests {
     use super::*;