Complete channel-closing behaviour

We now emit appropriate signals and return a non-error status code when
a channel is closed.

src/padfoot/channel/channel.rs

@@ -19,7 +19,7 @@ impl telepathy::Channel for Channel {
             .send(DbusAction::CloseChannel(self.path()))
             .unwrap();
 
-        Err(MethodErr::no_arg())
+        Ok(())
     }
 
     // Deprecated