Make Message and Presence class final

In order to prevent users from trying to subclass those two, trying to
add their custom attributes (or attribute values) in the element of
those two, instead of using extension elements aka. PacketExtension.

See

- http://stackoverflow.com/q/26516386/
- http://stackoverflow.com/q/26507753/
- http://stackoverflow.com/q/26542601/

smack-core/src/main/java/org/jivesoftware/smack/packet/Message.java

@@ -51,7 +51,7 @@ import org.jivesoftware.smack.util.XmlStringBuilder;
  *
  * @author Matt Tucker
  */
-public class Message extends Packet {
+public final class Message extends Packet {
 
     public static final String ELEMENT = "message";
     public static final String BODY = "body";

smack-core/src/main/java/org/jivesoftware/smack/packet/Presence.java

@@ -55,7 +55,7 @@ import org.jivesoftware.smack.util.XmlStringBuilder;
  * @see RosterPacket
  * @author Matt Tucker
  */
-public class Presence extends Packet {
+public final class Presence extends Packet {
 
     public static final String ELEMENT = "presence";