It is now possible to ask for "any number of history stanzas" when
joining a muc (with history=None).
Also we use "maxchars" when asking NO history ("0") since it's a MUST in
the XEP.
And you can specify a message when leaving a MUC.
* added stanza unhandled (unhandled iqs now reply with feature-not-implemented)
* added stanza exceptions (stanzas may now reply with exceptions when their handler raises an exception)
* sleekxmpp no longer spawns threads for callback handlers -- there are now two threads: one for handlers and one for reading. callback handlers can get results from the read queue directly with the "wait" handler which is used in .send() for the reply catching argument.
