New feature: Conversation backfilling

[jeremyckahn]

I've just shipped #22, which implements conversation backfilling. Now when you a join a room that already had participants, the conversation transcript will be pulled from the previous participants to get the new participant up to speed. As always, conversations are erased from a peers' local device when they leave the room. When all participants leave, the conversation is not held in memory by any participant and therefore erased from existence.

You can learn more at https://chitchatter.im/about. Let me know what you think!