Support for group invites

[moyamo]

As defined in XEP-0045 (Mediated Invites). With the protocol buffers I think we could reuse ConversationMessage and just have a different type (e.g. TYPE_ROOM_INVITE)
This could be translated to:

<message
    from='[email protected]'
    id='nzd143v8'
    to='[email protected]'>
  <x xmlns='http://jabber.org/protocol/muc#user'>
    <invite from='[email protected]/desktop'>
      <reason>
        Hey Hecate, this is the place for all good witches!
      </reason>
    </invite>
    <password>cauldronburn</password>
  </x>
</message>

ConversationMessage:
    userName: [email protected]
    buddyName: [email protected]
    nickname: [email protected]
    message: Hey Hecate, this is the place for all good witches!
    timestamp: <timestamp>
    headline: cauldronburn
    id: nzd143v8

[jprjr]

This would be very helpful. Some XMPP clients don't have a good/straightforward way to browse conferences. If I receive a group message from a legacy network, it would be very useful to receive an invite if I'm not already in the room.

[vitalyster]

If I understand issue correctly, this tries to solve the same issue as #68. Duplicate of #68