Stanza objects now accept the use of underscored names.
The CamelCase versions are still available for backwards compatibility,
but are discouraged.
The property stanza.values now maps to the old getStanzaValues and
setStanzaValues, in addition to _set_stanza_values and
_get_stanza_values.
The Roster stanza object builds item elements manually, and did not handle the
case where the name attribute is set to None, which would crash SleekXMPP.
* 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)
