support for OMEMO encryption

as requested in this thread

http://forum.miranda-ng.org/index.php?topic=4378.0

here is the ticket :)

http://conversations.im/omemo/
https://en.wikipedia.org/wiki/OMEMO_%28encryption%29

gajim already got a plugin. code here:
https://github.com/python-otr/gajim-otr/issues/18