Add OMEMO Encryption support to XMPP

Please add support for the new OMEMO draft-XEP:

OMEMO is an XMPP Extension Protocol (XEP) for secure multi-client end-to-end encryption: It offers Forward Secrecy and deniability while allowing you to keep the benefits of message synchronization and offline delivery.

OMEMO uses the Axolotl ratchet to establish secure sessions between every combination of devices:

Here is a corresponding ticket in Gajim's trac: