New karma module.

This module supports upvoting/downvoting topics in channels. Topics are
generally 1-20 characters in length, and can represent almost anything.

For example, karma voting typically happens when users contribute useful
or funny things to the channel. Others can then give them karma (similar to
reddit), or take away karma if they are being distruptive.

Features:
* Simple voting - `+1 [topic]` (or -1) is all it takes to vote.
* Voting limiter - users may only distribute karma every 2 minutes
* No self voting - if someone tries to upvote themselves, it becomes a minus
* Ability to check stats. 
  * ` .karma` prints your own username's karma
  * `.karma <topic>` shows the topics karma
  * `.rank` command shows x winning and losing topics
* Last reference voting. If a user posts something insightful, a `+1` with
   no topic references the last person who spoke. Similarly for a `-1`.