Added new way to calculate the probability an alternative is the winner

I added a new way to calculate the probability that a given alternative is the best. This is important for experiments with multiple alternatives, as the current z-score testing only compares the alternatives to the control and not to each other. The new feature simulates 10,000 draws from a probability distribution given each alternative's past performance, and then outputs the probability that it is the winner, given the rest of the alternatives.