Port group messages logic

[adambabik]

As a developer, I would like to be able to send group messages using the status-protocol-go so that I can communicate with other clients.

The current implementation is in status-react.

Acceptance criteria

The proper order is to be defined.

• [x] in v1/ add CreatePrivateGroupTextMessage function (#76)
• [x] implement encoder/decoder for GroupMembershipUpdate message (#76)
• [x] extend messageProcessor with SendGroup method
• [x] make sure messageProcessor.Process properly handles group messages
• [x] make sure messageProcessor.Process properly handles group membership updates
• [x] port membership updates validation logic
• [ ] port sending membership updates logic
• [ ] handle membership updates logic
• [ ] in Whisper transport add methods to set up proper topic filters
• [ ] in Messenger.Init setup proper filters based on active group chats

cc @cammellos

[adambabik]

Payload example: https://gist.github.com/cammellos/8621121434b2a470ff0fe1f3443929b2