Cleanup carbons, forwarded and a few others API

Adopt to common design patterns in Smack:

- getFrom(Packet) in Packetextensions
- INSTANCES.put() in getInstanceFor()
- ELEMENT instead of ELEMENT_NAME
- Use XmlStringBuilder

smack-extensions/src/test/java/org/jivesoftware/smackx/forward/ForwardedTest.java

@@ -51,7 +51,7 @@ public class ForwardedTest {
         fwd = (Forwarded) new ForwardedProvider().parseExtension(parser);
 
         // no delay in packet
-        assertEquals(null, fwd.getDelayInfo());
+        assertEquals(null, fwd.getDelayInformation());
         
         // check message
         assertEquals("romeo@montague.com", fwd.getForwardedPacket().getFrom());