* Generate device w/ keys
* PubSub set urn:xmpp:omemo:1:devices to announce new device
* PubSub set urn:xmpp:omemo:1:bundles to announce new key bundles
* PubSub get urn:xmpp:omemo:1:bundles to get user bundles

Sending a message:
* Create urn:xmpp:sce:0 with padding and content
* Add <encrypted xmlns='urn:xmpp:omemo:1'> with header (key list) and payload (b64)

Receiving a message:
* Check <keys> => <key>
* Decrypt (TODO)