Remove extra whitespace in AbstractDebugger
Add MultiUserChat.toString()
Use createMessage() where possible in MultiUserChat
Add support for *optional* session establishment

Don't perform XMPP session binding if the server announced the feature
as optional.

Fixes SMACK-622
Add ChatManager.createChat(String)

Also some minor refactors in Chat and changes in ChatManager.
Invoke MUC invitation listener async

Basically the same fix as 191c4429da6f2982d80c80bef81d7902a9640e6d was
for file transfer listeners, just for MUC invitation listeners.