Allow people to buy feedback points

We've added premium (paid) feedback points. The pricing model is interesting. We've decided to let the market decide how much a feedback point is worth. So we made the pricing dynamic. It depends on how many sites are currently in the review queue and the "velocity" of reviews being done. The main idea is to allow a reasonable turnaround time for sites who have bought premium feedback points. So the price goes up exponentially as there are more sites and less reviewers, and drops (again exponentially) if there are not too many sites in the queue. Ah... math :)