Fix CVE-2015-8688, and a few bugfixes.

slixmpp is vulnerable to roster push attacks as described by Daniel
Gultsch at https://gultsch.de/gajim_roster_push_and_message_interception.html.

(CVE-2015-8688)

(probably a past typo)

(typo)

Fix a few bugs along with the testsuite, and remove the asyncio loop
monkeypatch hack.

This allowed us to schedule events in-order later in the event loop, but
was detrimental to using other event loops and debugging.

They were using deprecated (and-removed) style.

Re-add relevant stanza methods, broken in 7cd1cf32

it breaks assumptions on the type of the value