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Fixes #267

Now we dont need to supply the account

- create a OMEMOConnection class to mimic more how Gajim does things

We would receive a KeyError for jids we dont have in the roster,
now we skip them and print a warning.

Fixes #173

The HTTPUploadPlugin calls that if encryption is enabled

Inactive or not known devices are added to the list
of possible receiving devices after we receive them
with a PreKeyWhisperMessage or WhisperMessage

This is so we dont have to rely only on devicelist
updates from the server