What steps will reproduce the problem?
1. User A sees a comment from user B and wants to make a follow-up comment.
2. User C has a private Jaiku account and User B comments on a post made by her (this should be 
invisible to the user A).
3. User A tries to post "@userb: I'd like to follow up on that..."
4. User A is met with a "Operation not allowed" response from the bot.

Ideally, Jaikuengine should iterate through user B's posts and comments starting with the most 
recent until it finds one that's available to user A. It should return a friendly error message upon 
reaching posts and comments that are older than a certain number of hours. The reply window 
should be configurable.