Remove "p instanceof Message" check

Smack already filters MUC PacketListeners for message types, ie. it's
guaranteed that those are Messages.
Remove unused import in JabberIMConnection
Improve JabberMUCMessageListener

Move the cast to Message to the beginning of processPacket(), since
Smack guarantees that MUC chat listeners are only called with Message
stanzas.

Don't iterate of the extensions, simply look-up if a DelayInformation
extension exists.
Same as in ea56fc5 just for the non-MUC message listener
Remove duplicate code in MessageListenerAdapters

by introducing AbstractJabberMessageListenerAdapter.
Switch to non-deprecated methods
Merge pull request #11 from Flowdalic/flow

Some housekeeping