getInstanceFor(connection) and getInstanceFor(connection, deviceid)
now throw NotLoggedInExceptions. When such an exception is thrown,
the client should react by using getInstanceFor(connection, jid)
or getInstanceFor(connection, jid, deviceId).
This commit adds the modules smack-omemo and smack-omemo-signal.
smack-omemo is licensed under the Apache license like the rest of the smack project.
smack-omemo-signal on the other hand is licensed under the GPLv3.
Due to the fact, that smack-omemo is not of much use without smack-omemo-signal,
the OMEMO feature can currently only be used by GPLv3 compatible software.
This may change in the future, when a more permissively licensed module becomes available.
Fixes SMACK-743.
