If a stanza handler raised an exception, the exception was processed
and replied by the modified stanza, not a stanza with the original
content.
A copy is now made before handler processing, and if an exception occurs
it is the copy that processes the exception using the original content.
This allows exceptions to include the original
content of a stanza in the error response by including
the parameter clear=False when raising the exception.
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.
* 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)
