* added stanza.keys()
* stanza.getValues() now return substanzas and plugins
* stanza.setValues() now can read substanzas and plugins
* stanzas can now be iterable if stanza.subitem is set to a class
* 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)