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

src/delay.rs

@@ -4,6 +4,7 @@
 // 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;
 
 use jid::Jid;
@@ -26,6 +27,8 @@ generate_element!(
     )
 );
 
+impl PresencePayload for Delay {}
+
 #[cfg(test)]
 mod tests {
     use super::*;