Karma decay

Optional configuration to allow karma modifications to decay over time and migrate back toward zero. The interval of time over which the decay happens is configurable.

When starting with pre-existing karma scores the handler fakes up some actions based on the score and the users who had previously modified the term. The faked actions will decay slowly and that rate will accelerate slowly. The method used to fake actions and their time to live is configurable, though it is really only used on the first start.

These changes are highly dependent upon, and currently branched from Pull #8.