Remove Pong class, add NAMESPACE to Ping

The Pong class was harmful, as people could try to use it with
PacketTypeFilter, which wouldn't work, a Pong is just a plain IQ result
without child XML.
Merge branch 'master' into xep198
Add log statement to roster result listener
Fix wrong boolean condition in async API

inverse boolean condition, correct is: "if packetListener has been
removed, then we received no response" and add check for
exceptionCallback being null.
Merge branch 'master' into xep198
Hide AbstractEnable class
Remove empty line
Fix IAE message in OnceForThisSTanza
Add AfterXStanzas.resetCounter()
Add ShortcutPredicates