Include some historical data to see how a score changes over time

This probably doesn't need to be too detailed, just something to give a glimpse of how a file's bugginess is changing over time.  I imagine doing a run over the repo from 6 months ago and another over the repo from 1 month ago.  Then in the UI give an indication for how a file is trending or what it's score was at those points in the past.  This was Dennis' idea.
