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.
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.
Make AbstractDelayInformationProvider handle empty string as reason