This was XEP-0237, but is now part of RFC 6121.
Roster backends should now expose two additional methods:
version(jid):
Return the version of the given JID's roster.
set_version(jid, version):
Update the version of the given JID's roster.
A new state field will be passed to the backend if an item
has been marked for removal. This is 'removed' which will
be set to True.
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)
