E2E encryption for private chats

[TatankaConCube]

The pull request describes how the E2E encryption can be integrated into your app. It DOESN'T cover the feature fully and is provided only to demonstrate one of the ways how it can be implemented using the ConneCtycube platform. So, it has the next limits:

1. The key exchange happens only in case the opponent is online during the creation chat (if the opponent is offline the key exchange can't be achieved and the messages will not be encrypted);
2. The E2E encryption applies for Private chats only;

The first feature can be resolved in the real app using the Push notifications feature for key exchange. The second feature requires encoding messages for every group member and sending them in one message. Then every member should take the message just for him and decode it.